february 1, 2021 · 2021. 2. 2. · for a non-zero fractional ideal i as in (1) above, the order of...

Northern Illinois University, Math 681

February 1, 2021

Math 681, Monday, February 1 February 1, 2021

More on Fractional Ideals

As has been our practice so far, K will be a number field with ring ofintegers OK . The upper case script German (“fraktur”) font will be usedto denote fractional ideals and the lower case Greek font will be used todenote elements of K .

Recall our big result.

Theorem (Fundamental Theorem)

The set of non-zero fractional ideals of K is a free abelian group on(generated by) the maximal ideals of OK .

In particular, any non-zero ideal I can be expressed uniquely as a productof non-zero prime ideals:

I = Pe11 · · ·P

err . (1)

As has been our practice so far,

K will be a number field with ring ofintegers OK . The upper case script German (“fraktur”) font will be usedto denote fractional ideals and the lower case Greek font will be used todenote elements of K .

I = Pe11 · · ·P

err . (1)

As has been our practice so far, K will be a number field with ring ofintegers OK .

The upper case script German (“fraktur”) font will be usedto denote fractional ideals and the lower case Greek font will be used todenote elements of K .

I = Pe11 · · ·P

err . (1)

I = Pe11 · · ·P

err . (1)

I = Pe11 · · ·P

err . (1)

I = Pe11 · · ·P

err . (1)

I = Pe11 · · ·P

err . (1)

I = Pe11 · · ·P

err . (1)

For two non-zero ideals A and B

we are on firm ground saying A|B ifB = AC for some non-zero ideal C by the Fundamental Theorem. Notethat A|B if and only if A ⊇ B as sets.

Definition

For a non-zero fractional ideal I as in (1) above, the order of I at themaximal ideal Pi is ei for i = 1, . . . , r . For all other maximal ideals P, theorder of I at P is 0. The order of OK at P is 0 for all maximal ideals P.We write ordP(I) for the order of I at P.

By the Fundamental Theorem, any ideal I is completely determined by theset of numbers

{ordP(I) : P a maximal ideal of OK}.

For two non-zero ideals A and B we are on firm ground saying A|B ifB = AC for some non-zero ideal C by the Fundamental Theorem.

Notethat A|B if and only if A ⊇ B as sets.

Definition

For two non-zero ideals A and B we are on firm ground saying A|B ifB = AC for some non-zero ideal C by the Fundamental Theorem. Notethat A|B if and only if A ⊇ B as sets.

Definition

For a non-zero fractional ideal I as in (1) above,

the order of I at themaximal ideal Pi is ei for i = 1, . . . , r . For all other maximal ideals P, theorder of I at P is 0. The order of OK at P is 0 for all maximal ideals P.We write ordP(I) for the order of I at P.

Definition

For a non-zero fractional ideal I as in (1) above, the order of I at themaximal ideal Pi is ei for i = 1, . . . , r .

For all other maximal ideals P, theorder of I at P is 0. The order of OK at P is 0 for all maximal ideals P.We write ordP(I) for the order of I at P.

Definition

For a non-zero fractional ideal I as in (1) above, the order of I at themaximal ideal Pi is ei for i = 1, . . . , r . For all other maximal ideals P,

theorder of I at P is 0. The order of OK at P is 0 for all maximal ideals P.We write ordP(I) for the order of I at P.

Definition

For a non-zero fractional ideal I as in (1) above, the order of I at themaximal ideal Pi is ei for i = 1, . . . , r . For all other maximal ideals P, theorder of I at P is 0.

The order of OK at P is 0 for all maximal ideals P.We write ordP(I) for the order of I at P.

Definition

For a non-zero fractional ideal I as in (1) above, the order of I at themaximal ideal Pi is ei for i = 1, . . . , r . For all other maximal ideals P, theorder of I at P is 0. The order of OK at P is 0 for all maximal ideals P.

We write ordP(I) for the order of I at P.

Definition

Given two non-zero ideals A and B,

we define the greatest common divisorgcd(A,B) and least common multiple lcm(A,B) of A and B as follows:

(gcd(A,B)

)= min{ordP(A), ordP(B)},

(lcm(A,B)

)= max{ordP(A), ordP(B)}

for all maximal ideals P. We say A and B are relatively prime if theirgreatest common divisor is OK .

We will abuse notation just a bit here and extend all of the above toindividual non-zero elements α ∈ K via principal ideals. For example,ordP(α) = ordP

((α)), where (α) is the principal ideal generated by α.

Definition

Given two non-zero ideals A and B, we define the greatest common divisorgcd(A,B)

and least common multiple lcm(A,B) of A and B as follows:

(gcd(A,B)

(lcm(A,B)

Definition

Given two non-zero ideals A and B, we define the greatest common divisorgcd(A,B) and least common multiple lcm(A,B) of A and B as follows:

(gcd(A,B)

(lcm(A,B)

Definition

(gcd(A,B)

(lcm(A,B)

Definition

(gcd(A,B)

(lcm(A,B)

Definition

(gcd(A,B)

(lcm(A,B)

for all maximal ideals P.

We say A and B are relatively prime if theirgreatest common divisor is OK .

Definition

(gcd(A,B)

(lcm(A,B)

Definition

(gcd(A,B)

(lcm(A,B)

We will abuse notation just a bit here and extend all of the above toindividual non-zero elements α ∈ K via principal ideals.

For example,ordP(α) = ordP

Definition

(gcd(A,B)

(lcm(A,B)

Note that the gcd(A,B) is the smallest (set-theoretically) ideal whichcontains both A and B.

In other words,

gcd(A,B) = A + B := {α + β : α ∈ A, β ∈ B}.

Similarly, the lcm(A,B) is the largest (set-theoretically) ideal which iscontained in both A and B. It isn’t difficult to see thatgcd(A,B) lcm(A,B) = AB.

Note that the gcd(A,B) is the smallest (set-theoretically) ideal whichcontains both A and B. In other words,

gcd(A,B) = A + B := {α + β : α ∈ A, β ∈ B}.

Similarly, the lcm(A,B) is the largest (set-theoretically) ideal which iscontained in both A and B.

It isn’t difficult to see thatgcd(A,B) lcm(A,B) = AB.

gcd(A,B) = A + B := {α + β : α ∈ A, β ∈ B}.

Clearly ordP(AB) = ordP(A) + ordP(B).

Since A + B = gcd(A,B), wehave ordP(A + B) = min{ordP(A), ordP(B)}. However, it is not generallythe case that (α) + (β) = (α + β) for α, β ∈ OK . Since(α) + (β)|(α + β), we do have

ordP(α + β) ≥ min{ordP(α), ordP(β)}.

One can check that this is an equality whenever ordP(α) 6= ordP(β).

Lemma (1)

Let A be a non-zero ideal and α ∈ OK \ {0}. Then there is a non-zeroideal B with AB = (α) if and only if α ∈ A.

Proof: By the Fundamental Theorem AB = (α) if and only ifB = (α)A−1, and (α)A−1 ⊆ OK if and only if (α) ⊆ A.

Clearly ordP(AB) = ordP(A) + ordP(B). Since A + B = gcd(A,B), wehave ordP(A + B) = min{ordP(A), ordP(B)}.

However, it is not generallythe case that (α) + (β) = (α + β) for α, β ∈ OK . Since(α) + (β)|(α + β), we do have

Lemma (1)

Clearly ordP(AB) = ordP(A) + ordP(B). Since A + B = gcd(A,B), wehave ordP(A + B) = min{ordP(A), ordP(B)}. However, it is not generallythe case that (α) + (β) = (α + β) for α, β ∈ OK .

Since(α) + (β)|(α + β), we do have

Lemma (1)

Clearly ordP(AB) = ordP(A) + ordP(B). Since A + B = gcd(A,B), wehave ordP(A + B) = min{ordP(A), ordP(B)}. However, it is not generallythe case that (α) + (β) = (α + β) for α, β ∈ OK . Since(α) + (β)|(α + β),

we do have

Lemma (1)

Clearly ordP(AB) = ordP(A) + ordP(B). Since A + B = gcd(A,B), wehave ordP(A + B) = min{ordP(A), ordP(B)}. However, it is not generallythe case that (α) + (β) = (α + β) for α, β ∈ OK . Since(α) + (β)|(α + β), we do have

Lemma (1)

Let A be a non-zero ideal and α ∈ OK \ {0}.

Then there is a non-zeroideal B with AB = (α) if and only if α ∈ A.

Lemma (1)

Proof:

By the Fundamental Theorem AB = (α) if and only ifB = (α)A−1, and (α)A−1 ⊆ OK if and only if (α) ⊆ A.

Lemma (1)

Proof: By the Fundamental Theorem AB = (α) if and only ifB = (α)A−1,

and (α)A−1 ⊆ OK if and only if (α) ⊆ A.

Lemma (1)

Lemma (2)

Let A and B be non-zero ideals. Then there is an α ∈ A withgcd

((α),AB

Proof: This is obvious if A = OK (just use α = 1), so assume A 6= OK .Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB. To ease notation here, let ei = ordPi

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

ej + 1 ≥ 1 otherwise.

Thus, gcd(A1, . . . ,Ar ) = OK , which implies that there are αi ∈ Ai fori = 1, . . . , r with

α1 + · · ·+ αr = 1. (2)

Lemma (2)

Let A and B be non-zero ideals.

Then there is an α ∈ A withgcd

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

Proof:

This is obvious if A = OK (just use α = 1), so assume A 6= OK .Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB. To ease notation here, let ei = ordPi

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

Proof: This is obvious if A = OK

(just use α = 1), so assume A 6= OK .Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB. To ease notation here, let ei = ordPi

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

Proof: This is obvious if A = OK (just use α = 1),

so assume A 6= OK .Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB. To ease notation here, let ei = ordPi

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

Proof: This is obvious if A = OK (just use α = 1), so assume A 6= OK .

Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB. To ease notation here, let ei = ordPi

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

Proof: This is obvious if A = OK (just use α = 1), so assume A 6= OK .Let P1, . . . ,Pr be the maximal ideals occurring in the unique factorizationof AB.

To ease notation here, let ei = ordPi(A) for i = 1, . . . , r .

DefineAi = AP1 · · ·PrP

−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .

DefineAi = AP1 · · ·PrP

−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

Thus, gcd(A1, . . . ,Ar ) = OK ,

which implies that there are αi ∈ Ai fori = 1, . . . , r with

α1 + · · ·+ αr = 1. (2)

Lemma (2)

((α),AB

(A) for i = 1, . . . , r .Define

Ai = AP1 · · ·PrP−ei−1i , i = 1, . . . , r .

Note that

ordPj(Ai ) =

{0 if i = j ,

α1 + · · ·+ αr = 1. (2)

Since each αi ∈ Ai we have

ordPj(αi ) ≥ ordPj

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

Since ordP(1) = 0 for all maximal ideals P, (2) and (3) imply that

ordPi(αi ) = 0, i = 1, . . . , r . (4)

Now chose βi ∈ Peii \P

ei+1i for all i = 1, . . . , r and let

α = α1β1 + · · ·+ αrβr .

By construction we have ordPi(βi ) = ei for all i = 1, . . . , r . This together

with (3) and (4) show that

ordPi(α) = ei , i = 1, . . . , r .

Since ordP(AB) = 0 for all P not among P1, . . . ,Pr , we havegcd

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

Since ordP(1) = 0 for all maximal ideals P,

(2) and (3) imply that

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

ei+1i for all i = 1, . . . , r

and let

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

By construction we have ordPi(βi ) = ei for all i = 1, . . . , r .

This togetherwith (3) and (4) show that

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

Since ordP(AB) = 0 for all P not among P1, . . . ,Pr ,

we havegcd

((α),AB

(Ai ) = ej + 1 ≥ 1 i 6= j . (3)

ordPi(αi ) = 0, i = 1, . . . , r . (4)

α = α1β1 + · · ·+ αrβr .

ordPi(α) = ei , i = 1, . . . , r .

((α),AB

Combining Lemmas 1 and 2 gives us the following result.

Lemma (3)

Let A be a non-zero ideal and let β ∈ A \ {0}. Then there is an α ∈ Awith gcd(α, β) = A. In particular, all non-zero ideals can be viewed as thegreatest common divisor of two algebraic integers.

We can speak of congruences in OK in much the same way we do in Z.Specifically, for a non-zero ideal A and α, β ∈ OK , we say α is congruentto β modulo A if α− β ∈ A. We denote this more compactly by writingα ≡ β mod A. A more “advanced” way to say this is α + A = β + A aselements of the quotient ring OK/A.

Lemma (3)

Let A be a non-zero ideal and let β ∈ A \ {0}.

Then there is an α ∈ Awith gcd(α, β) = A. In particular, all non-zero ideals can be viewed as thegreatest common divisor of two algebraic integers.

Lemma (3)

Let A be a non-zero ideal and let β ∈ A \ {0}. Then there is an α ∈ Awith gcd(α, β) = A.

In particular, all non-zero ideals can be viewed as thegreatest common divisor of two algebraic integers.

Lemma (3)

We can speak of congruences in OK in much the same way we do in Z.

Specifically, for a non-zero ideal A and α, β ∈ OK , we say α is congruentto β modulo A if α− β ∈ A. We denote this more compactly by writingα ≡ β mod A. A more “advanced” way to say this is α + A = β + A aselements of the quotient ring OK/A.

Lemma (3)

We can speak of congruences in OK in much the same way we do in Z.Specifically, for a non-zero ideal A and α, β ∈ OK ,

we say α is congruentto β modulo A if α− β ∈ A. We denote this more compactly by writingα ≡ β mod A. A more “advanced” way to say this is α + A = β + A aselements of the quotient ring OK/A.

Lemma (3)

We can speak of congruences in OK in much the same way we do in Z.Specifically, for a non-zero ideal A and α, β ∈ OK , we say α is congruentto β modulo A if α− β ∈ A.

We denote this more compactly by writingα ≡ β mod A. A more “advanced” way to say this is α + A = β + A aselements of the quotient ring OK/A.

Lemma (3)

We can speak of congruences in OK in much the same way we do in Z.Specifically, for a non-zero ideal A and α, β ∈ OK , we say α is congruentto β modulo A if α− β ∈ A. We denote this more compactly by writingα ≡ β mod A.

A more “advanced” way to say this is α + A = β + A aselements of the quotient ring OK/A.

Lemma (3)

The existence of solutions to linear congruences is very much the same asit is with Z.

Lemma (4)

Let A be a non-zero ideal and let α, β ∈ OK . Then the congruence

Xα ≡ β mod A

has a solution in OK if and only if gcd((α),A

)|(β).

Proof: This congruence has a solution if and only if β ∈ A + (α), that is,(β) ⊆ gcd

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

((α),A

Lemma (4)

Let A be a non-zero ideal and let α, β ∈ OK .

Then the congruence

Xα ≡ β mod A

)|(β).

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

Proof:

This congruence has a solution if and only if β ∈ A + (α), that is,(β) ⊆ gcd

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

Proof: This congruence has a solution if and only if β ∈ A + (α),

that is,(β) ⊆ gcd

((α),A

Lemma (4)

Xα ≡ β mod A

)|(β).

((α),A

We also know when we can solve simultaneous congruences.

Theorem (Chinese Remainder Theorem)

Let A1, . . . ,Ar be non-zero ideals which are pair-wise relatively prime, i.e.,Ai + Aj = OK whenever i 6= j . Let I denote the product A1 · · ·Ar . Then

OK/I ∼= OK/A1 × · · · ×OK/Ar .

In particular, given β1, . . . , βr ∈ OK there is an α ∈ OK with