rainbow colourings - portal · → λ. y ⊂ x is an f-rainbowiff f ↾ y n is 1–1 . problem...
TRANSCRIPT
![Page 1: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/1.jpg)
Rainbow Colourings
Lajos Soukup
Alfréd Rényi Institute of MathematicsHungarian Academy of Sciences
http://www.renyi.hu/ ∼soukup
Logic Colloquium 2008
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 1 / 36
![Page 2: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/2.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 3: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/3.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 4: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/4.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 5: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/5.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 6: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/6.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 7: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/7.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 8: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/8.jpg)
The beginnings
Ramsey theory : find large homogeneous sets
monochromatic sets
anti Ramsey theory : find large inhomogeneous sets
polychromatic sets, rainbow sets
first anti Ramsey theorems : Rado, 1973.
polychromatic Ramsey, rainbow Ramsey
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 2 / 36
![Page 9: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/9.jpg)
The beginnings
Definition
Let f :[
X]n
→ λ.Y ⊂ X is an f -rainbow iff f ↾
[
Y]n is 1–1 .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 3 / 36
![Page 10: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/10.jpg)
The beginnings
Definition
Let f :[
X]n
→ λ.Y ⊂ X is an f -rainbow iff f ↾
[
Y]n is 1–1 .
X
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 3 / 36
![Page 11: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/11.jpg)
The beginnings
Definition
Let f :[
X]n
→ λ.Y ⊂ X is an f -rainbow iff f ↾
[
Y]n is 1–1 .
Y
X
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 3 / 36
![Page 12: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/12.jpg)
The beginnings
Definition
Let f :[
X]n
→ λ.Y ⊂ X is an f -rainbow iff f ↾
[
Y]n is 1–1 .
Y
X
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 3 / 36
![Page 13: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/13.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 14: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/14.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 15: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/15.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
ω1X
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 16: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/16.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
ω1X
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 17: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/17.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
ω1X
Notation
f establishes that ω1 6→[
ω1]2
3:
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 18: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/18.jpg)
The beginnings
Let f :[
X]n
→ λ. Y ⊂ X is an f -rainbow iff f ↾[
Y]n
is 1–1 .
Problem (Erdos, Hajnal, 1967, Problem 68)
Assume that f :[
ω1]2
→ 3 establishes ω1 6→[
ω1]2
3.Does there exist an f -rainbow triangle ?
ω1X
Notation
f establishes that ω1 6→[
ω1]2
3: f ⊢ ω1 6→[
ω1]2
3
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 4 / 36
![Page 19: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/19.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 20: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/20.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 21: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/21.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
f ⊢ ω1 6→[
(ω, ω1)]2
3
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 22: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/22.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
f ⊢ ω1 6→[
(ω, ω1)]2
3 iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1
ω1A B
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 23: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/23.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
f ⊢ ω1 6→[
(ω, ω1)]2
3 iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = 3.
ω1A B
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 24: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/24.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 25: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/25.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
X Y
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 26: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/26.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
X Y
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 27: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/27.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
X YΦ
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 28: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/28.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
X YΦ
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 29: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/29.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
X YΦ
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 30: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/30.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 31: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/31.jpg)
Rainbow triangle
Assume f ⊢ ω1 6→[
ω1]2
3. Does there exist an f -rainbow triangle ?
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2
3 then there is a rainbow triangle .
Definition
Let d :[
X]2
→ λ, f :[
Y]2
→ λ. f realizes d (d =⇒ f )
iff ∃Φ : X 1–1−→ Y s.t. ∀{ζ, ξ} ∈
[
X]2 d(ζ, ξ) = f (Φ(ζ),Φ(ξ)).
Fact
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 5 / 36
![Page 32: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/32.jpg)
Rainbow triangle
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Theorem (Shelah, 1975)
CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle.
♦ =⇒ ∃f ⊢ ω1 6→[
ω1]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 6 / 36
![Page 33: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/33.jpg)
Rainbow triangle
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Theorem (Shelah, 1975)
CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle.
♦ =⇒ ∃f ⊢ ω1 6→[
ω1]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 6 / 36
![Page 34: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/34.jpg)
Rainbow triangle
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Theorem (Shelah, 1975)
CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle.
♦ =⇒ ∃f ⊢ ω1 6→[
ω1]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 6 / 36
![Page 35: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/35.jpg)
Rainbow triangle
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Theorem (Shelah, 1975)
CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle.
♦ =⇒ ∃f ⊢ ω1 6→[
ω1]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 6 / 36
![Page 36: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/36.jpg)
Rainbow triangle
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Theorem (Shelah, 1975)
CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle.
♦ =⇒ ∃f ⊢ ω1 6→[
ω1]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 6 / 36
![Page 37: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/37.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 38: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/38.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 39: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/39.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 40: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/40.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 41: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/41.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 42: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/42.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 43: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/43.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
First try:
By transfinite induction on α < ω1 define f (ξ, α) for ξ < α.
First challenge : ω1 6→[
ω1]2ω
fix a |• -sequence {Aζ : ζ < ω1} ⊂
[
ω1]ω
if ζ < α and Aζ ⊂ α then f ′′[Aζ , {α}] = ω
Second challenge : no f -rainbow triangle ???
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 7 / 36
![Page 44: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/44.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 45: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/45.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 46: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/46.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 47: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/47.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
∆
x y ∆(x , y)ω
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 48: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/48.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
∆
x y ∆(x , y)ω
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 49: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/49.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
∆
x y ∆(x , y)ω ω
h
k
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 50: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/50.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
∆
x y ∆(x , y)ω ω
h
k
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 51: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/51.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
Second try: Fix the colouring first! Colour[
2ω]2 as follows:
∆(x , y) = min{n : x(n) 6= y(n)}. No ∆-rainbow triangle!
h : ω → ω s.t. h−1{k} is infinite for each k ∈ ω.
f (x , y) = h(∆(x , y)). No f -rainbow triangle!
2ω
∆
x y ∆(x , y)ω ω
h
k
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 8 / 36
![Page 52: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/52.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
f (x , y) = h(∆(x , y)). No f -rainbow triangle.
Choose S = {sα : α < ω1} ⊂ 2ω s .t. f ↾[
S]2 works.
Enumerate[
ω1]ω as {Aξ : ξ < ω1}. Write Sξ = {sν : ν ∈ Aξ}.
if ξ < α and sα ∈ Sξ then f ′′[{α}, Sξ ] = ω.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 9 / 36
![Page 53: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/53.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
f (x , y) = h(∆(x , y)). No f -rainbow triangle.
Choose S = {sα : α < ω1} ⊂ 2ω s .t. f ↾[
S]2 works.
Enumerate[
ω1]ω as {Aξ : ξ < ω1}. Write Sξ = {sν : ν ∈ Aξ}.
if ξ < α and sα ∈ Sξ then f ′′[{α}, Sξ ] = ω.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 9 / 36
![Page 54: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/54.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
f (x , y) = h(∆(x , y)). No f -rainbow triangle.
Choose S = {sα : α < ω1} ⊂ 2ω s .t. f ↾[
S]2 works.
Enumerate[
ω1]ω as {Aξ : ξ < ω1}. Write Sξ = {sν : ν ∈ Aξ}.
if ξ < α and sα ∈ Sξ then f ′′[{α}, Sξ ] = ω.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 9 / 36
![Page 55: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/55.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
f (x , y) = h(∆(x , y)). No f -rainbow triangle.
Choose S = {sα : α < ω1} ⊂ 2ω s .t. f ↾[
S]2 works.
Enumerate[
ω1]ω as {Aξ : ξ < ω1}. Write Sξ = {sν : ν ∈ Aξ}.
if ξ < α and sα ∈ Sξ then f ′′[{α}, Sξ ] = ω.Sξ sα
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 9 / 36
![Page 56: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/56.jpg)
Proof: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω, no f -rainbow triangle
f (x , y) = h(∆(x , y)). No f -rainbow triangle.
Choose S = {sα : α < ω1} ⊂ 2ω s .t. f ↾[
S]2 works.
Enumerate[
ω1]ω as {Aξ : ξ < ω1}. Write Sξ = {sν : ν ∈ Aξ}.
if ξ < α and sα ∈ Sξ then f ′′[{α}, Sξ ] = ω.
2ω
Sξ sα ω ω
h
k
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 9 / 36
![Page 57: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/57.jpg)
A weaker partition relation
Fact: If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
f ⊢ ω1 6→[
(ω, ω1)]2ω1
iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = ω1.
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 10 / 36
![Page 58: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/58.jpg)
A weaker partition relation
Fact: If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
f ⊢ ω1 6→[
(ω, ω1)]2ω1
iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = ω1.
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 10 / 36
![Page 59: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/59.jpg)
A weaker partition relation
Fact: If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
f ⊢ ω1 6→[
(ω, ω1)]2ω1
iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = ω1.
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
A
B
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 10 / 36
![Page 60: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/60.jpg)
A weaker partition relation
Fact: If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
f ⊢ ω1 6→[
(ω, ω1)]2ω1
iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = ω1.
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
A
B
α
β
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 10 / 36
![Page 61: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/61.jpg)
A weaker partition relation
Fact: If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
f ⊢ ω1 6→[
(ω, ω1)]2ω1
iff ∀A ∈[
ω1]ω
∀B ∈[
ω1]ω1 f ′′[A, B] = ω1.
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
A
B
α
β
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 10 / 36
![Page 62: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/62.jpg)
A weaker partition relation
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Proof: If A, B0, B1, · · · ∈[
ω1]ω1 and ν0, ν1, · · · ∈ ω1 then ∃α ∈ A,
∃B′i ∈
[
Bi]ω1 such that f ′′[B′
i , {α}] = {νi}.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 11 / 36
![Page 63: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/63.jpg)
A weaker partition relation
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Proof: If A, B0, B1, · · · ∈[
ω1]ω1 and ν0, ν1, · · · ∈ ω1 then ∃α ∈ A,
∃B′i ∈
[
Bi]ω1 such that f ′′[B′
i , {α}] = {νi}.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 11 / 36
![Page 64: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/64.jpg)
A weaker partition relation
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Proof: If A, B0, B1, · · · ∈[
ω1]ω1 and ν0, ν1, · · · ∈ ω1 then ∃α ∈ A,
∃B′i ∈
[
Bi]ω1 such that f ′′[B′
i , {α}] = {νi}.
A B0 B1 Bn
ν0 ν1 νn
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 11 / 36
![Page 65: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/65.jpg)
A weaker partition relation
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Proof: If A, B0, B1, · · · ∈[
ω1]ω1 and ν0, ν1, · · · ∈ ω1 then ∃α ∈ A,
∃B′i ∈
[
Bi]ω1 such that f ′′[B′
i , {α}] = {νi}.
A B0 B1 Bn
B′0
B′1 B′
nαν0 ν1 νn
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 11 / 36
![Page 66: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/66.jpg)
A weaker partition relation
Theorem (Erdos, Hajnal, 1978)
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Proof: If A, B0, B1, · · · ∈[
ω1]ω1 and ν0, ν1, · · · ∈ ω1 then ∃α ∈ A,
∃B′i ∈
[
Bi]ω1 such that f ′′[B′
i , {α}] = {νi}.
A B0 B1 Bn
B′0
B′1 B′
nαν0 ν1 νn
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 11 / 36
![Page 67: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/67.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2
ω1then f realizes each function d :
[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 68: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/68.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 69: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/69.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
A
B
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 70: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/70.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
A
B
α
β
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 71: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/71.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
A
B
α
β
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 72: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/72.jpg)
One more partition relation
If f ⊢ ω1 6→[
(ω1;ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν and α < β.
Problem
Assume f ⊢ ω1 6→[
(ω1,ω1)]2ω1
. Does f realize each function
d :[
ω]2
→ ω1?
∀A, B ∈[
ω1]ω1 ∀ν < ω1 ∃α ∈ A ∃β ∈ B f (α, β) = ν
A
B
α
β
ν
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 12 / 36
![Page 73: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/73.jpg)
A new method
Assume f ⊢ ω1 6→[
(ω1,ω1)]2
ω1. Does f realize each function d :
[
ω]2
→ ω1?
Fact
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Todorcevic)
ω1 6→[
ω1]2ω1
.
Theorem (Shelah)
ω2 6→[
(ω2;ω2)]2ω2
.
Theorem (Moore)
ω1 6→[
(ω1;ω1)]2ω1
.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 13 / 36
![Page 74: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/74.jpg)
A new method
Assume f ⊢ ω1 6→[
(ω1,ω1)]2
ω1. Does f realize each function d :
[
ω]2
→ ω1?
Fact
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Todorcevic)
ω1 6→[
ω1]2ω1
.
Theorem (Shelah)
ω2 6→[
(ω2;ω2)]2ω2
.
Theorem (Moore)
ω1 6→[
(ω1;ω1)]2ω1
.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 13 / 36
![Page 75: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/75.jpg)
A new method
Assume f ⊢ ω1 6→[
(ω1,ω1)]2
ω1. Does f realize each function d :
[
ω]2
→ ω1?
Fact
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Todorcevic)
ω1 6→[
ω1]2ω1
.
Theorem (Shelah)
ω2 6→[
(ω2;ω2)]2ω2
.
Theorem (Moore)
ω1 6→[
(ω1;ω1)]2ω1
.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 13 / 36
![Page 76: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/76.jpg)
A new method
Assume f ⊢ ω1 6→[
(ω1,ω1)]2
ω1. Does f realize each function d :
[
ω]2
→ ω1?
Fact
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Todorcevic)
ω1 6→[
ω1]2ω1
.
Theorem (Shelah)
ω2 6→[
(ω2;ω2)]2ω2
.
Theorem (Moore)
ω1 6→[
(ω1;ω1)]2ω1
.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 13 / 36
![Page 77: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/77.jpg)
A new method
Assume f ⊢ ω1 6→[
(ω1,ω1)]2
ω1. Does f realize each function d :
[
ω]2
→ ω1?
Fact
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Todorcevic)
ω1 6→[
ω1]2ω1
.
Theorem (Shelah)
ω2 6→[
(ω2;ω2)]2ω2
.
Theorem (Moore)
ω1 6→[
(ω1;ω1)]2ω1
.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 13 / 36
![Page 78: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/78.jpg)
Recent results
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 14 / 36
![Page 79: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/79.jpg)
Recent results
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 14 / 36
![Page 80: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/80.jpg)
Recent results
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 14 / 36
![Page 81: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/81.jpg)
Recent results
If f ⊢ ω1 6→[
(ω1, ω1)]2ω1
then d =⇒ f for each d :[
3]2
→ ω1.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 14 / 36
![Page 82: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/82.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 83: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/83.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 84: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/84.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 85: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/85.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 86: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/86.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 87: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/87.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
A
Bω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 88: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/88.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
A
B
A′
B′
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 89: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/89.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}
A
B
A′
B′
ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 90: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/90.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 91: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/91.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 92: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/92.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 93: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/93.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 94: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/94.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 95: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/95.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 96: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/96.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 97: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/97.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 98: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/98.jpg)
∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 ∃d :[
5]2 1–1
−→ 10 d 6=⇒ f .
Let g :[
ω1]2
→ 2 be a Sierpinski colouring: {rα : α < ω1} ⊂ R; forα < β < ω1 let g(α, β) = 1 iff rα < rβ .Let e :
[
5]2
→ 2 be the “pentagon without diagonals”: e(α, β) = 1iff α ≡ β + 1 mod 5
(⋆) e 6=⇒ g.(†) ∀A, B ∈
[
ω1]ω1 ∃A′ ∈
[
A]ω1 ∃B′ ∈
[
B]ω1 ∃i < 2
g′′[A′; B′] = {i} and g′′[B′; A′] = {1 − i}Let m ⊢ ω1 6→
[
(ω1;ω1)]2
5Put f (α, β) = 2m(α, β) + g(α, β).Assume that A, B ∈
[
ω1]ω1. Let ℓ = 2n + i < 10.
∃A′ ∈[
A]ω1 ∃B′ ∈
[
B]ω1 g′′[A′; B′] = {i} (or g′′[B′; A′] = {i})
∃α ∈ A′ ∃β ∈ B′ α < β and m(α, β) = n.f (α, β) = 2n + i = ℓ.∃m′ :
[
5]2
→ 5 s.t. d(α, β) = 2m′(α, β) + e(α, β) is 1–1.if d =⇒ f then d mod 2 =⇒ f mod 2, i.e. e =⇒ g.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 15 / 36
![Page 99: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/99.jpg)
Recent results
[Hajnal] ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Theorem (Hajnal)
If f ⊢ ω1 6→[
(ω1, ω1)]2ω
then there exists an infinite f -rainbow set .
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 16 / 36
![Page 100: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/100.jpg)
Recent results
[Hajnal] ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10 s.t. d 6=⇒ f for some rainbow d :[
5]2
→ 10.
Theorem (Hajnal)
If f ⊢ ω1 6→[
(ω1, ω1)]2ω
then there exists an infinite f -rainbow set .
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 16 / 36
![Page 101: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/101.jpg)
Recent results
Shelah:CH =⇒ ∃f ⊢ ω1 6→
[
ω1]2ω
, no f -rainbow triangle.
♦ =⇒ ∃g ⊢ ω1 6→[
ω1]2ω1
, no g-rainbow triangle.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 17 / 36
![Page 102: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/102.jpg)
Recent results
Shelah:CH =⇒ ∃f ⊢ ω1 6→
[
ω1]2
ω, no f -rainbow triangle.
♦ =⇒ ∃g ⊢ ω1 6→[
ω1]2
ω1, no g-rainbow triangle.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 17 / 36
![Page 103: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/103.jpg)
Recent results
Shelah:CH =⇒ ∃f ⊢ ω1 6→
[
ω1]2
ω, no f -rainbow triangle.
♦ =⇒ ∃g ⊢ ω1 6→[
ω1]2
ω1, no g-rainbow triangle.
Theorem (Hajnal)
∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 17 / 36
![Page 104: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/104.jpg)
Stepping up:colourings of ω2
Negative theorems
Shelah: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2ω
, no f -rainbow triangle.
Theorem (Shelah)
CH + 2ω1 = ω2 =⇒ ∃f ⊢ ω2 6→[
ω2]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 18 / 36
![Page 105: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/105.jpg)
Stepping up:colourings of ω2
Negative theorems
Shelah: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2
ω, no f -rainbow triangle.
Theorem (Shelah)
CH + 2ω1 = ω2 =⇒ ∃f ⊢ ω2 6→[
ω2]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 18 / 36
![Page 106: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/106.jpg)
Stepping up:colourings of ω2
Negative theorems
Shelah: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2
ω, no f -rainbow triangle.
Theorem (Shelah)
CH + 2ω1 = ω2 =⇒ ∃f ⊢ ω2 6→[
ω2]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 18 / 36
![Page 107: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/107.jpg)
Stepping up:colourings of ω2
Negative theorems
Shelah: CH =⇒ ∃f ⊢ ω1 6→[
ω1]2
ω, no f -rainbow triangle.
Theorem (Shelah)
CH + 2ω1 = ω2 =⇒ ∃f ⊢ ω2 6→[
ω2]2ω1
, no f -rainbow triangle.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 18 / 36
![Page 108: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/108.jpg)
Stepping up:colourings of ω2
Hajnal: ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10s.t. d 6=⇒ f for some rainbow d :
[
5]2
→ 10.
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
(ω2, ω2)]2
10 s.t. d 6=⇒ f for some rainbow
d :[
5]2
→ 10.
Hajnal: ∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
ω2]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 19 / 36
![Page 109: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/109.jpg)
Stepping up:colourings of ω2
Hajnal: ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10s.t. d 6=⇒ f for some rainbow d :
[
5]2
→ 10.
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
(ω2, ω2)]2
10 s.t. d 6=⇒ f for some rainbow
d :[
5]2
→ 10.
Hajnal: ∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
ω2]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 19 / 36
![Page 110: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/110.jpg)
Stepping up:colourings of ω2
Hajnal: ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10s.t. d 6=⇒ f for some rainbow d :
[
5]2
→ 10.
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
(ω2, ω2)]2
10 s.t. d 6=⇒ f for some rainbow
d :[
5]2
→ 10.
Hajnal: ∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
ω2]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 19 / 36
![Page 111: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/111.jpg)
Stepping up:colourings of ω2
Hajnal: ∃f ⊢ ω1 6→[
(ω1, ω1)]2
10s.t. d 6=⇒ f for some rainbow d :
[
5]2
→ 10.
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
(ω2, ω2)]2
10 s.t. d 6=⇒ f for some rainbow
d :[
5]2
→ 10.
Hajnal: ∃f ⊢ ω1 6→[
ω1]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Theorem
CH =⇒ ∃f ⊢ ω2 6→[
ω2]2
6 and ∃d :[
4]2
→ 6 rainbow s.t. d 6=⇒ f .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 19 / 36
![Page 112: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/112.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 113: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/113.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 114: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/114.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 115: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/115.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 116: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/116.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 117: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/117.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 118: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/118.jpg)
Stepping up:colourings of ω2
Positive theorems
If f ⊢ ω1 6→[
(ω, ω1)]2ω1
then f realizes each function d :[
ω]2
→ ω1.
Assume f ⊢ ω2 6→[
(ω1, ω2)]2
2. Does f realize each function
d :[
ω1]2
→ 2?
Theorem (Shelah, 1975)
(a) If f ⊢ ω2 6→[
(ω1;ω2)]2
2 then V Fn(ω,2) |= f ⊢ ω2 6→[
(ω1;ω2)]2
2.
(b) In V Fn(ω,2) there is a colouring c :[
ω1]2
→ 2 such that c 6=⇒ f forany f ∈ V.
(c) It is consistent that ∃f ⊢ ω2 6→[
(ω1;ω2)]2
2 such that c 6=⇒ f for some
c :[
ω1]2
→ 2.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 20 / 36
![Page 119: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/119.jpg)
Colourings of ω2
Problem (Erdos, Hajnal, 1978)
Assume GCH and f ⊢ ω2 6→(ω1.+ ω)2
2. Does f realize each function
f :[
ω1]2
→ 2?
Theorem (Hajnal)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1;ω)]2ω1).
∀A ∈[
ω2]ω1 ∀B ∈
[
ω2]ω if sup A < min B then f ′′[A, B] = ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 21 / 36
![Page 120: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/120.jpg)
Colourings of ω2
Problem (Erdos, Hajnal, 1978)
Assume GCH and f ⊢ ω2 6→(ω1.+ ω)2
2. Does f realize each function
f :[
ω1]2
→ 2?
Theorem (Hajnal)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1;ω)]2ω1).
∀A ∈[
ω2]ω1 ∀B ∈
[
ω2]ω if sup A < min B then f ′′[A, B] = ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 21 / 36
![Page 121: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/121.jpg)
Colourings of ω2
Problem (Erdos, Hajnal, 1978)
Assume GCH and f ⊢ ω2 6→(ω1.+ ω)2
2. Does f realize each function
f :[
ω1]2
→ 2?
Theorem (Hajnal)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1;ω)]2ω1).
∀A ∈[
ω2]ω1 ∀B ∈
[
ω2]ω if sup A < min B then f ′′[A, B] = ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 21 / 36
![Page 122: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/122.jpg)
Colourings of ω2
Problem (Erdos, Hajnal, 1978)
Assume GCH and f ⊢ ω2 6→(ω1.+ ω)2
2. Does f realize each function
f :[
ω1]2
→ 2?
Theorem (Hajnal)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1;ω)]2ω1).
∀A ∈[
ω2]ω1 ∀B ∈
[
ω2]ω if sup A < min B then f ′′[A, B] = ω1
ω2A B
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 21 / 36
![Page 123: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/123.jpg)
Colourings of ω2
Problem (Erdos, Hajnal, 1978)
Assume GCH and f ⊢ ω2 6→(ω1.+ ω)2
2. Does f realize each function
f :[
ω1]2
→ 2?
Theorem (Hajnal)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1;ω)]2ω1).
∀A ∈[
ω2]ω1 ∀B ∈
[
ω2]ω if sup A < min B then f ′′[A, B] = ω1
ω2A B
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 21 / 36
![Page 124: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/124.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 125: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/125.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 126: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/126.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 127: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/127.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 128: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/128.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 129: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/129.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 130: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/130.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 131: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/131.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 132: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/132.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 133: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/133.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Y X ω2
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 134: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/134.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Y X ω2Aβ
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 135: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/135.jpg)
Con ( GCH + ∃f ⊢ ω2 6→[(ω1; ω)]2ω1).
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].
Y X ω2Aβ
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 22 / 36
![Page 136: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/136.jpg)
Colourings of ω2
Problem of Erdos and Hajnal:Assume GCH and f ⊢ ω2 6→[(ω1; ω)]2ω1
. Does f realize every c :[
ω1]2
→ 2?
(Shelah): If f ⊢ ω2 6→[
(ω1; ω2)]2
2then V Fn(ω,2) |= f ⊢ ω2 6→
[
(ω1; ω2)]2
2.
Theorem
(a) If f ⊢ ω2 6→[(ω1;ω)]2ω1then V Fn(κ,2) |= “ f ⊢ ω2 6→[(ω1;ω)]2ω1
” .
(b) Con (GCH +∃f ⊢ ω2 6→[
(ω1;ω)]2ω1
and ∃c :[
ω1]2
→ 2 s.t. c 6=⇒ f .)
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 23 / 36
![Page 137: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/137.jpg)
Colourings of ω2
Problem of Erdos and Hajnal:Assume GCH and f ⊢ ω2 6→[(ω1; ω)]2ω1
. Does f realize every c :[
ω1]2
→ 2?
(Shelah): If f ⊢ ω2 6→[
(ω1; ω2)]2
2then V Fn(ω,2) |= f ⊢ ω2 6→
[
(ω1; ω2)]2
2.
Theorem
(a) If f ⊢ ω2 6→[(ω1;ω)]2ω1then V Fn(κ,2) |= “ f ⊢ ω2 6→[(ω1;ω)]2ω1
” .
(b) Con (GCH +∃f ⊢ ω2 6→[
(ω1;ω)]2ω1
and ∃c :[
ω1]2
→ 2 s.t. c 6=⇒ f .)
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 23 / 36
![Page 138: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/138.jpg)
Colourings of ω2
Problem of Erdos and Hajnal:Assume GCH and f ⊢ ω2 6→[(ω1; ω)]2ω1
. Does f realize every c :[
ω1]2
→ 2?
(Shelah): If f ⊢ ω2 6→[
(ω1; ω2)]2
2then V Fn(ω,2) |= f ⊢ ω2 6→
[
(ω1; ω2)]2
2.
Theorem
(a) If f ⊢ ω2 6→[(ω1;ω)]2ω1then V Fn(κ,2) |= “ f ⊢ ω2 6→[(ω1;ω)]2ω1
” .
(b) Con (GCH +∃f ⊢ ω2 6→[
(ω1;ω)]2ω1
and ∃c :[
ω1]2
→ 2 s.t. c 6=⇒ f .)
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 23 / 36
![Page 139: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/139.jpg)
Colourings of ω2
Problem of Erdos and Hajnal:Assume GCH and f ⊢ ω2 6→[(ω1; ω)]2ω1
. Does f realize every c :[
ω1]2
→ 2?
(Shelah): If f ⊢ ω2 6→[
(ω1; ω2)]2
2then V Fn(ω,2) |= f ⊢ ω2 6→
[
(ω1; ω2)]2
2.
Theorem
(a) If f ⊢ ω2 6→[(ω1;ω)]2ω1then V Fn(κ,2) |= “ f ⊢ ω2 6→[(ω1;ω)]2ω1
” .
(b) Con (GCH +∃f ⊢ ω2 6→[
(ω1;ω)]2ω1
and ∃c :[
ω1]2
→ 2 s.t. c 6=⇒ f .)
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 23 / 36
![Page 140: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/140.jpg)
Colourings of ω2
Problem of Erdos and Hajnal:Assume GCH and f ⊢ ω2 6→[(ω1; ω)]2ω1
. Does f realize every c :[
ω1]2
→ 2?
(Shelah): If f ⊢ ω2 6→[
(ω1; ω2)]2
2then V Fn(ω,2) |= f ⊢ ω2 6→
[
(ω1; ω2)]2
2.
Theorem
(a) If f ⊢ ω2 6→[(ω1;ω)]2ω1then V Fn(κ,2) |= “ f ⊢ ω2 6→[(ω1;ω)]2ω1
” .
(b) Con (GCH +∃f ⊢ ω2 6→[
(ω1;ω)]2ω1
and ∃c :[
ω1]2
→ 2 s.t. c 6=⇒ f .)
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 23 / 36
![Page 141: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/141.jpg)
Rainbow on ω2
Problem (Hajnal)
Assume GCH + f ⊢ ω2 6→[(ω1;ω)]2ω1. Does there exist an uncountable
f -rainbow set?
Theorem
It is consistent that GCH holds and there is a function f :[
ω2]2
→ ω1
such that
(1) f ⊢ ω2 6→[(ω1;ω)]2ω1,
(2) there is no uncountable f -rainbow.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 24 / 36
![Page 142: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/142.jpg)
Rainbow on ω2
Problem (Hajnal)
Assume GCH + f ⊢ ω2 6→[(ω1;ω)]2ω1. Does there exist an uncountable
f -rainbow set?
Theorem
It is consistent that GCH holds and there is a function f :[
ω2]2
→ ω1
such that
(1) f ⊢ ω2 6→[(ω1;ω)]2ω1,
(2) there is no uncountable f -rainbow.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 24 / 36
![Page 143: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/143.jpg)
Rainbow on ω2
Problem (Hajnal)
Assume GCH + f ⊢ ω2 6→[(ω1;ω)]2ω1. Does there exist an uncountable
f -rainbow set?
Theorem
It is consistent that GCH holds and there is a function f :[
ω2]2
→ ω1
such that
(1) f ⊢ ω2 6→[(ω1;ω)]2ω1,
(2) there is no uncountable f -rainbow.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 24 / 36
![Page 144: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/144.jpg)
Rainbow on ω2
Problem (Hajnal)
Assume GCH + f ⊢ ω2 6→[(ω1;ω)]2ω1. Does there exist an uncountable
f -rainbow set?
Theorem
It is consistent that GCH holds and there is a function f :[
ω2]2
→ ω1
such that
(1) f ⊢ ω2 6→[(ω1;ω)]2ω1,
(2) there is no uncountable f -rainbow.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 24 / 36
![Page 145: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/145.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 146: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/146.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 147: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/147.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 148: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/148.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 149: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/149.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 150: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/150.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 151: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/151.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 152: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/152.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 153: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/153.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 154: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/154.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 155: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/155.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 156: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/156.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 157: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/157.jpg)
Proof: ∃f ⊢ ω2 6→[(ω1; ω)]2ω1, no uncountable rainbow
First try:
Assume CH. Define P = 〈P,�〉 σ-complete, ω2-c.c.Underlying set : 〈X , c,A, ξ〉
X ∈[
ω2]ω
, c :[
X]2
→ ω1
A ⊂ P(X), |A| ≤ ω,ξ ∈ ω1
Ordering: 〈Y , d ,B, ζ〉 � 〈X , c,A, ξ〉 iffY ⊃ X ,d ⊃ c, B ⊃ A, ζ ≥ ξ
(1) ∀A ∈ A ∀β ∈ (Y \ X) ∩ min A ξ ⊂ d ′′[{β}, A].(2) Seal certain countable rainbow sets !
∀A ∈ A ∀β ∈ (Y \ X) the set A ∪ {β} is not a d-rainbow .
〈P,�〉 does not satisfy ω2-c.c.!
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 25 / 36
![Page 158: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/158.jpg)
Colouring of 2ω1
It is consistent that GCH holds and ∃f ⊢ ω2 6→[(ω1; ω)]2ω1s.t. there is no un-
countable f -rainbow.
TheoremIt is consistent that CH holds, 2ω1 is arbitrarily large and∃g ⊢ 2ω1 6→[(ω1, ω2)]
2ω1
s. t. there is no uncountable g-rainbow subsetof 2ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 26 / 36
![Page 159: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/159.jpg)
Colouring of 2ω1
It is consistent that GCH holds and ∃f ⊢ ω2 6→[(ω1; ω)]2ω1s.t. there is no un-
countable f -rainbow.
TheoremIt is consistent that CH holds, 2ω1 is arbitrarily large and∃g ⊢ 2ω1 6→[(ω1, ω2)]
2ω1
s. t. there is no uncountable g-rainbow subsetof 2ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 26 / 36
![Page 160: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/160.jpg)
Colouring of 2ω1
It is consistent that GCH holds and ∃f ⊢ ω2 6→[(ω1; ω)]2ω1s.t. there is no un-
countable f -rainbow.
TheoremIt is consistent that CH holds, 2ω1 is arbitrarily large and∃g ⊢ 2ω1 6→[(ω1, ω2)]
2ω1
s. t. there is no uncountable g-rainbow subsetof 2ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 26 / 36
![Page 161: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/161.jpg)
Colouring of 2ω1
It is consistent that GCH holds and ∃f ⊢ ω2 6→[(ω1; ω)]2ω1s.t. there is no un-
countable f -rainbow.
TheoremIt is consistent that CH holds, 2ω1 is arbitrarily large and∃g ⊢ 2ω1 6→[(ω1, ω2)]
2ω1
s. t. there is no uncountable g-rainbow subsetof 2ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 26 / 36
![Page 162: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/162.jpg)
Colouring of 2ω1
It is consistent that GCH holds and ∃f ⊢ ω2 6→[(ω1; ω)]2ω1s.t. there is no un-
countable f -rainbow.
TheoremIt is consistent that CH holds, 2ω1 is arbitrarily large and∃g ⊢ 2ω1 6→[(ω1, ω2)]
2ω1
s. t. there is no uncountable g-rainbow subsetof 2ω1.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 26 / 36
![Page 163: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/163.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 164: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/164.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 165: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/165.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 166: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/166.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 167: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/167.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 168: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/168.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 169: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/169.jpg)
The →∗ relation
f :[
X]n
→ C is κ-bounded iff |f−1{c}| ≤ κ for each c ∈ C.
λ →∗ (α)nκ−bdd iff for every κ-bounded colouring of
[
λ]n there is a
rainbow set of order type α,
→poly
Theorem (Galvin)
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 27 / 36
![Page 170: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/170.jpg)
λ → (α)nκ implies λ →∗ (α)n
κ−bdd
Proof:
Let f :[
λ]n
→ λ be κ-bounded
There is a function g :[
λ]n
→ κ such that: if f (A) = f (B) theng(A) 6= g(B)
If Y ⊂ λ is g-monochromatic then Y is f -rainbow
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 28 / 36
![Page 171: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/171.jpg)
λ → (α)nκ implies λ →∗ (α)n
κ−bdd
Proof:
Let f :[
λ]n
→ λ be κ-bounded
There is a function g :[
λ]n
→ κ such that: if f (A) = f (B) theng(A) 6= g(B)
If Y ⊂ λ is g-monochromatic then Y is f -rainbow
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 28 / 36
![Page 172: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/172.jpg)
λ → (α)nκ implies λ →∗ (α)n
κ−bdd
Proof:
Let f :[
λ]n
→ λ be κ-bounded
There is a function g :[
λ]n
→ κ such that: if f (A) = f (B) theng(A) 6= g(B)
If Y ⊂ λ is g-monochromatic then Y is f -rainbow
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 28 / 36
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λ → (α)nκ implies λ →∗ (α)n
κ−bdd
Proof:
Let f :[
λ]n
→ λ be κ-bounded
There is a function g :[
λ]n
→ κ such that: if f (A) = f (B) theng(A) 6= g(B)
If Y ⊂ λ is g-monochromatic then Y is f -rainbow
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 28 / 36
![Page 174: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/174.jpg)
λ → (α)nκ implies λ →∗ (α)n
κ−bdd
Proof:
Let f :[
λ]n
→ λ be κ-bounded
There is a function g :[
λ]n
→ κ such that: if f (A) = f (B) theng(A) 6= g(B)
If Y ⊂ λ is g-monochromatic then Y is f -rainbow
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 28 / 36
![Page 175: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/175.jpg)
Basic theorems on →∗
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Corollary(Galvin)
ω1 →∗ (α)22−bdd for α < ω1.
Theorem (Galvin)
CH implies that ω1 6→∗(ω1)
22−bdd .
Theorem (Todorcevic)
PFA implies that ω1 →∗ (ω1)22−bdd .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 29 / 36
![Page 176: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/176.jpg)
Basic theorems on →∗
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Corollary(Galvin)
ω1 →∗ (α)22−bdd for α < ω1.
Theorem (Galvin)
CH implies that ω1 6→∗(ω1)
22−bdd .
Theorem (Todorcevic)
PFA implies that ω1 →∗ (ω1)22−bdd .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 29 / 36
![Page 177: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/177.jpg)
Basic theorems on →∗
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Corollary(Galvin)
ω1 →∗ (α)22−bdd for α < ω1.
Theorem (Galvin)
CH implies that ω1 6→∗(ω1)
22−bdd .
Theorem (Todorcevic)
PFA implies that ω1 →∗ (ω1)22−bdd .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 29 / 36
![Page 178: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/178.jpg)
Basic theorems on →∗
λ → (α)nk implies λ →∗ (α)n
k−bdd.
Corollary(Galvin)
ω1 →∗ (α)22−bdd for α < ω1.
Theorem (Galvin)
CH implies that ω1 6→∗(ω1)
22−bdd .
Theorem (Todorcevic)
PFA implies that ω1 →∗ (ω1)22−bdd .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 29 / 36
![Page 179: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/179.jpg)
Under Martin’s Axiom
If c :[
ω1]2
→ ω1 is 2-bounded then T.F.A.E.
c ⊢ ω1 6→∗(ω1)
22−bdd
there is no uncountable c-rainbow
Theorem (Abraham, Cummings, Smyth)
It is consistent that there is a function c :[
ω1]2
→ ω1 whichc.c.c-indestructibly establishes ω1 6→
∗(ω1)22−bdd .
If CH holds and there is a Suslin-tree then there is a functionc′ :
[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 30 / 36
![Page 180: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/180.jpg)
Under Martin’s Axiom
If c :[
ω1]2
→ ω1 is 2-bounded then T.F.A.E.
c ⊢ ω1 6→∗(ω1)
22−bdd
there is no uncountable c-rainbow
Theorem (Abraham, Cummings, Smyth)
It is consistent that there is a function c :[
ω1]2
→ ω1 whichc.c.c-indestructibly establishes ω1 6→
∗(ω1)22−bdd .
If CH holds and there is a Suslin-tree then there is a functionc′ :
[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 30 / 36
![Page 181: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/181.jpg)
Under Martin’s Axiom
If c :[
ω1]2
→ ω1 is 2-bounded then T.F.A.E.
c ⊢ ω1 6→∗(ω1)
22−bdd
there is no uncountable c-rainbow
Theorem (Abraham, Cummings, Smyth)
It is consistent that there is a function c :[
ω1]2
→ ω1 whichc.c.c-indestructibly establishes ω1 6→
∗(ω1)22−bdd .
If CH holds and there is a Suslin-tree then there is a functionc′ :
[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 30 / 36
![Page 182: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/182.jpg)
Under Martin’s Axiom
If c :[
ω1]2
→ ω1 is 2-bounded then T.F.A.E.
c ⊢ ω1 6→∗(ω1)
22−bdd
there is no uncountable c-rainbow
Theorem (Abraham, Cummings, Smyth)
It is consistent that there is a function c :[
ω1]2
→ ω1 whichc.c.c-indestructibly establishes ω1 6→
∗(ω1)22−bdd .
If CH holds and there is a Suslin-tree then there is a functionc′ :
[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 30 / 36
![Page 183: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/183.jpg)
Under Martin’s Axiom
If c :[
ω1]2
→ ω1 is 2-bounded then T.F.A.E.
c ⊢ ω1 6→∗(ω1)
22−bdd
there is no uncountable c-rainbow
Theorem (Abraham, Cummings, Smyth)
It is consistent that there is a function c :[
ω1]2
→ ω1 whichc.c.c-indestructibly establishes ω1 6→
∗(ω1)22−bdd .
If CH holds and there is a Suslin-tree then there is a functionc′ :
[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 30 / 36
![Page 184: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/184.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 185: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/185.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 186: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/186.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 187: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/187.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 188: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/188.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 189: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/189.jpg)
Under Martin’s Axiom
Con ( ∃c :[
ω1]2
→ ω1 s.t for each c.c.c P V P |= f ⊢ ω1 6→∗(ω1)
22−bdd .)
Con( ∃c′ :[
ω1]2
→ 2 and there is a c.c.c poset Q such that
(a) V |= there is no uncountable c′-rainbow set,
(b) V Q |= there is an uncountable c′-rainbow set.
TheoremIf GCH holds then for each k ∈ ω there is a k-bounded colouringf :
[
ω1]2
→ ω1 and there are two c.c.c posets P and Q such thatVP |= “f c.c.c-indestructibly establishes ω1 6→
∗[(ω1;ω1)]k−bdd ”,but
VQ |= “ ω1 is the union of countably many f -rainbow sets ”.
f is k-bounded,for each A ∈
[
ω1]ω1 and B ∈
[
ω1]ω1 there is ξ such that
|{〈α, β〉 ∈ A × B : α < β ∧ f (α, β) = ξ}| = k .
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 31 / 36
![Page 190: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/190.jpg)
A black box theorem
Based on some results of Abraham and Todorcevic.
Fnm(ω1, K ) = {s : s is a function, dom(s) ∈[
ω1]m
, ran(s) ⊂ K}
〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) is dom-disjoint iff dom(sα)∩ dom(sβ) = ∅
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 32 / 36
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A black box theorem
Based on some results of Abraham and Todorcevic.
Fnm(ω1, K ) = {s : s is a function, dom(s) ∈[
ω1]m
, ran(s) ⊂ K}
〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) is dom-disjoint iff dom(sα)∩ dom(sβ) = ∅
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 32 / 36
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A black box theorem
Based on some results of Abraham and Todorcevic.
Fnm(ω1, K ) = {s : s is a function, dom(s) ∈[
ω1]m
, ran(s) ⊂ K}
〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) is dom-disjoint iff dom(sα)∩ dom(sβ) = ∅
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 32 / 36
![Page 193: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/193.jpg)
A black box theorem
Based on some results of Abraham and Todorcevic.
Fnm(ω1, K ) = {s : s is a function, dom(s) ∈[
ω1]m
, ran(s) ⊂ K}
〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) is dom-disjoint iff dom(sα)∩ dom(sβ) = ∅
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 32 / 36
![Page 194: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/194.jpg)
A black box theorem
DefinitionLet G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if givenany dom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there areα < β < ω1 such that
[sα, sβ ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
K
ω1
G
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 33 / 36
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A black box theorem
DefinitionLet G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if givenany dom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there areα < β < ω1 such that
[sα, sβ ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
K
ω1
sαsβ
sγ
dom(sα) dom(sβ) dom(sγ)
G
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 33 / 36
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A black box theorem
DefinitionLet G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if givenany dom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there areα < β < ω1 such that
[sα, sβ ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
K
ω1
sαsβ
sγ
dom(sα) dom(sβ) dom(sγ)
G
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 33 / 36
![Page 197: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/197.jpg)
A black box theorem
DefinitionLet G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if givenany dom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there areα < β < ω1 such that
[sα, sβ ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
K
ω1
sαsβ
sγ
dom(sα) dom(sβ) dom(sγ)
G
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 33 / 36
![Page 198: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/198.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 199: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/199.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 200: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/200.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 201: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/201.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 202: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/202.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 203: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/203.jpg)
A blackbox theorem
Let G be a graph on ω1 × K , m ∈ ω. We say that G is m-solid if given anydom-disjoint sequence 〈sα : α < ω1〉 ⊂ Fnm(ω1, K ) there are α < β < ω1 suchthat
[sα, sβ] ⊂ G.
G is called strongly solid iff it is m-solid for each m ∈ ω.
TheoremAssume 2ω1 = ω2. If G is a strongly solid graph on ω1 × K , where|K | ≤ 2ω1 , then for each m ∈ ω there is a c.c.c poset P of size ω2 suchthat
V P |= “G is c.c.c-indestructibly m-solid. ”
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 34 / 36
![Page 204: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/204.jpg)
Bibliography
Abraham, U; Cummings, J; Smyth, C; Some results in polychromatic Ramsey theory. J.Symb. Log. 72, no. 3, 865-896 (2007)
Hajnal, A; Rainbow Ramsey Theorems for Colorings Establishing Negative PartitionRelations, Fund. Math. 198 (2008), 255-262.
Shelah, S; Coloring without triangles and partition relations, Israel J. Math. 20 (1975), 1-12
Soukup, L; Indestructible colourings and rainbow Ramsey theorems, arxiv preprint.
Todorcevic, S; Forcing positive partition relations. Trans. Am. Math. Soc. 280, 703-720(1983)
http://www.renyi.hu/∼soukup
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 35 / 36
![Page 205: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/205.jpg)
Bibliography
Abraham, U; Cummings, J; Smyth, C; Some results in polychromatic Ramsey theory. J.Symb. Log. 72, no. 3, 865-896 (2007)
Hajnal, A; Rainbow Ramsey Theorems for Colorings Establishing Negative PartitionRelations, Fund. Math. 198 (2008), 255-262.
Shelah, S; Coloring without triangles and partition relations, Israel J. Math. 20 (1975), 1-12
Soukup, L; Indestructible colourings and rainbow Ramsey theorems, arxiv preprint.
Todorcevic, S; Forcing positive partition relations. Trans. Am. Math. Soc. 280, 703-720(1983)
http://www.renyi.hu/∼soukup
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 35 / 36
![Page 206: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/206.jpg)
Summary
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36
![Page 207: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/207.jpg)
Summary
ZFC ω1 6→[
(ω, ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36
![Page 208: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/208.jpg)
Summary
ZFC ω1 6→[
(ω, ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
CH ω1 6→[
ω1]2ω
∧ no rainbow K3
♦ ω1 6→[
ω1]2ω1
∧ no rainbow K3
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36
![Page 209: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/209.jpg)
Summary
ZFC ω1 6→[
(ω, ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
ZFC ω1 6→[
(ω1;ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
CH ω1 6→[
ω1]2ω
∧ no rainbow K3
♦ ω1 6→[
ω1]2ω1
∧ no rainbow K3
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36
![Page 210: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/210.jpg)
Summary
ZFC ω1 6→[
(ω, ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
ZFC ω1 6→[
(ω1;ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
ZFC ω1 6→[
(ω1, ω1)]2ω1
=⇒ ∀d :[
3]2
→ ω1
ZFC ω1 6→[
(ω1, ω1)]2
10 ∧ not univ for K5-rainbows
ZFC ω1 6→[
(ω1, ω1)]2ω1
=⇒ ∃ infinite rainbow
CH ω1 6→[
ω1]2ω
∧ no rainbow K3
♦ ω1 6→[
ω1]2ω1
∧ no rainbow K3
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36
![Page 211: Rainbow Colourings - Portal · → λ. Y ⊂ X is an f-rainbowiff f ↾ Y n is 1–1 . Problem (Erdos, Hajnal, 1967, Problem 68)˝ Assume that f : ω1 2 → 3 establishes ω1 →](https://reader035.vdocument.in/reader035/viewer/2022071211/60242c75dd7b28634640814b/html5/thumbnails/211.jpg)
Summary
ZFC ω1 6→[
(ω, ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
ZFC ω1 6→[
(ω1;ω1)]2ω1
=⇒ ∀d :[
ω]2
→ ω1
ZFC ω1 6→[
(ω1, ω1)]2ω1
=⇒ ∀d :[
3]2
→ ω1
ZFC ω1 6→[
(ω1, ω1)]2
10 ∧ not univ for K5-rainbows
ZFC ω1 6→[
(ω1, ω1)]2ω1
=⇒ ∃ infinite rainbow
CH ω1 6→[
ω1]2ω
∧ no rainbow K3
♦ ω1 6→[
ω1]2ω1
∧ no rainbow K3
ZFC ω1 6→[
ω1]2
6 ∧ not univ for K4-rainbows
Soukup, L (Rényi Institute) Rainbow Colourings LC2008 36 / 36