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Conferencia Internacional en Memo-ria de Pilar Pison Casares – Sevilla 2008
Cremona Maps
as related to
Linear Syzygies and Ideals of Linear Type
(In collaboration with F. Russo)
Aron Simis
Universidade Federal de PernambucoRecife, Brazil
Conferencia en Memoria de Pilar Pison Casares – p. 1/50
Sample references
• I. Dolgachev, Polar Cremona transformations, Mich.
Math. J. 48 (2000), 191–202.
Conferencia en Memoria de Pilar Pison Casares – p. 2/50
Sample references
• I. Dolgachev, Polar Cremona transformations, Mich.
Math. J. 48 (2000), 191–202.
• C. Ciliberto, F. Russo and A. Simis, Homaloidalhypersurfaces and hypersurfaces with vanishing Hessian,arXiv:math.AG/0701596 21 Jan 2007.
Conferencia en Memoria de Pilar Pison Casares – p. 2/50
Sample references
• I. Dolgachev, Polar Cremona transformations, Mich.
Math. J. 48 (2000), 191–202.
• C. Ciliberto, F. Russo and A. Simis, Homaloidalhypersurfaces and hypersurfaces with vanishing Hessian,arXiv:math.AG/0701596 21 Jan 2007.
• L. Ein and N. Sheperd Barron, Some specialCremona transformations, Amer. J. Math. 111 (1989),
783-800.
Conferencia en Memoria de Pilar Pison Casares – p. 2/50
Sample references
• I. Dolgachev, Polar Cremona transformations, Mich.
Math. J. 48 (2000), 191–202.
• C. Ciliberto, F. Russo and A. Simis, Homaloidalhypersurfaces and hypersurfaces with vanishing Hessian,arXiv:math.AG/0701596 21 Jan 2007.
• L. Ein and N. Sheperd Barron, Some specialCremona transformations, Amer. J. Math. 111 (1989),
783-800.
• D. Eisenbud and B. Ulrich , Row Ideals and Fibers ofMorphisms, arXiv:math.AG/0705.3931 27 May 2007.
Conferencia en Memoria de Pilar Pison Casares – p. 2/50
Sample references
• I. Dolgachev, Polar Cremona transformations, Mich.
Math. J. 48 (2000), 191–202.
• C. Ciliberto, F. Russo and A. Simis, Homaloidalhypersurfaces and hypersurfaces with vanishing Hessian,arXiv:math.AG/0701596 21 Jan 2007.
• L. Ein and N. Sheperd Barron, Some specialCremona transformations, Amer. J. Math. 111 (1989),
783-800.
• D. Eisenbud and B. Ulrich , Row Ideals and Fibers ofMorphisms, arXiv:math.AG/0705.3931 27 May 2007.
• E. Guardo and B. Harbourne, Resolutions of idealsof six fat points inP
2, J. Algebra, to appear.Conferencia en Memoria de Pilar Pison Casares – p. 2/50
Sample references
• B. Harbourne, On Nagata’s conjecture, J. Algebra
236 (2001), 692-702.
Conferencia en Memoria de Pilar Pison Casares – p. 3/50
Sample references
• B. Harbourne, On Nagata’s conjecture, J. Algebra
236 (2001), 692-702.
• K. Hulek, S. Katz and F.-O. Schreyer, Cremonatransformations and syzygies, Math. Z. 209 (3)(1992),
419–443.
Conferencia en Memoria de Pilar Pison Casares – p. 3/50
Sample references
• B. Harbourne, On Nagata’s conjecture, J. Algebra
236 (2001), 692-702.
• K. Hulek, S. Katz and F.-O. Schreyer, Cremonatransformations and syzygies, Math. Z. 209 (3)(1992),
419–443.
• F. Russo and A. Simis, On birational maps andJacobian matrices, Compositio Math. 126 (2001),
335–358.
Conferencia en Memoria de Pilar Pison Casares – p. 3/50
Sample references
• B. Harbourne, On Nagata’s conjecture, J. Algebra
236 (2001), 692-702.
• K. Hulek, S. Katz and F.-O. Schreyer, Cremonatransformations and syzygies, Math. Z. 209 (3)(1992),
419–443.
• F. Russo and A. Simis, On birational maps andJacobian matrices, Compositio Math. 126 (2001),
335–358.
• A. Simis, Cremona transformations and some relatedalgebras, J. Algebra 280 (2004), 162–179.
Conferencia en Memoria de Pilar Pison Casares – p. 3/50
Sample references
• B. Harbourne, On Nagata’s conjecture, J. Algebra
236 (2001), 692-702.
• K. Hulek, S. Katz and F.-O. Schreyer, Cremonatransformations and syzygies, Math. Z. 209 (3)(1992),
419–443.
• F. Russo and A. Simis, On birational maps andJacobian matrices, Compositio Math. 126 (2001),
335–358.
• A. Simis, Cremona transformations and some relatedalgebras, J. Algebra 280 (2004), 162–179.
• A. Simis, B. Ulrich and W. Vasconcelos, Jacobiandual fibrations, Amer. J. Math. 115 (1993), 47–75.
Conferencia en Memoria de Pilar Pison Casares – p. 3/50
FIRST PART
ALGEBRAICPRELIMINARIES
Conferencia en Memoria de Pilar Pison Casares – p. 4/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Here SR(I) and RR(I) denote the symmetric and the Rees
algebra of I , respectively.
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Here SR(I) and RR(I) denote the symmetric and the Rees
algebra of I , respectively.
Behold:
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Here SR(I) and RR(I) denote the symmetric and the Rees
algebra of I , respectively.
Behold:
• If I contains a regular element then I being of linear type
means that SR(I) is a torsionfree R-algebra.
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Here SR(I) and RR(I) denote the symmetric and the Rees
algebra of I , respectively.
Behold:
• If I contains a regular element then I being of linear type
means that SR(I) is a torsionfree R-algebra.
• If I is of linear type then µ(IP ) ≤ htP for every prime
ideal P ⊃ I ; equivalently (if I contains a regular element):
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Ideals of linear type
An ideal I ⊂ R in a ring R is said to be of linear typeif the
natural R-algebra homomorphism SR(I) ։ RR(I) is
injective.
Here SR(I) and RR(I) denote the symmetric and the Rees
algebra of I , respectively.
Behold:
• If I contains a regular element then I being of linear type
means that SR(I) is a torsionfree R-algebra.
• If I is of linear type then µ(IP ) ≤ htP for every prime
ideal P ⊃ I ; equivalently (if I contains a regular element):
• ht It(ϕ) ≥ m+ 1 − t, 1 ≤ t ≤ m− 1, where
Rp ϕ−→ Rm −→ I −→ 0 is a free presentation.
Conferencia en Memoria de Pilar Pison Casares – p. 5/50
Rees algebras and their presentation
Throughout I 6= 0 is a homogeneous ideal in the standard
polynomial ring R = k[x] = k[x0, . . . , xn] over a field k.
We assume that I is generated by forms of one same degree
d ≥ 2; accordingly, one has a free graded presentation.
Conferencia en Memoria de Pilar Pison Casares – p. 6/50
Rees algebras and their presentation
Throughout I 6= 0 is a homogeneous ideal in the standard
polynomial ring R = k[x] = k[x0, . . . , xn] over a field k.
We assume that I is generated by forms of one same degree
d ≥ 2; accordingly, one has a free graded presentation.
R(−(d+1))ℓ⊕∑
j≥2R(−(d+j))ϕ
−→ R(−d)m → I → 0
Conferencia en Memoria de Pilar Pison Casares – p. 6/50
Rees algebras and their presentation
Throughout I 6= 0 is a homogeneous ideal in the standard
polynomial ring R = k[x] = k[x0, . . . , xn] over a field k.
We assume that I is generated by forms of one same degree
d ≥ 2; accordingly, one has a free graded presentation.
R(−(d+1))ℓ⊕∑
j≥2R(−(d+j))ϕ
−→ R(−d)m → I → 0
The image of R(−(d+ 1))ℓ by ϕ, denoted ϕ1, will be called
the linear part of ϕ.
Conferencia en Memoria de Pilar Pison Casares – p. 6/50
Rees algebras and their presentation
Throughout I 6= 0 is a homogeneous ideal in the standard
polynomial ring R = k[x] = k[x0, . . . , xn] over a field k.
We assume that I is generated by forms of one same degree
d ≥ 2; accordingly, one has a free graded presentation.
R(−(d+1))ℓ⊕∑
j≥2R(−(d+j))ϕ
−→ R(−d)m → I → 0
The image of R(−(d+ 1))ℓ by ϕ, denoted ϕ1, will be called
the linear part of ϕ.
We say that ϕ1 has maximal rank(or that ϕ has maximallinear rank) if rank(ϕ1) = rank(ϕ).
Conferencia en Memoria de Pilar Pison Casares – p. 6/50
Rees algebras and their presentation
Throughout I 6= 0 is a homogeneous ideal in the standard
polynomial ring R = k[x] = k[x0, . . . , xn] over a field k.
We assume that I is generated by forms of one same degree
d ≥ 2; accordingly, one has a free graded presentation.
R(−(d+1))ℓ⊕∑
j≥2R(−(d+j))ϕ
−→ R(−d)m → I → 0
The image of R(−(d+ 1))ℓ by ϕ, denoted ϕ1, will be called
the linear part of ϕ.
We say that ϕ1 has maximal rank(or that ϕ has maximallinear rank) if rank(ϕ1) = rank(ϕ).The maximal rank condition is trivially attained if ϕ1 = ϕ in
which case I is said to have linear presentationor is
linearly presentedConferencia en Memoria de Pilar Pison Casares – p. 6/50
Rees algebras and their presentation
Let J = ker (k[x, y] ։ R(I)) denote apresentation ideal of the Rees algebra ofthe ideal I ⊂ R = k[x].
Conferencia en Memoria de Pilar Pison Casares – p. 7/50
Rees algebras and their presentation
Let J = ker (k[x, y] ։ R(I)) denote apresentation ideal of the Rees algebra ofthe ideal I ⊂ R = k[x].
J bihomogeneous in the standard bigradingof k[x, y] as we are assuming that I isgenerated in fixed degree d.
Conferencia en Memoria de Pilar Pison Casares – p. 7/50
Rees algebras and their presentation
Let J = ker (k[x, y] ։ R(I)) denote apresentation ideal of the Rees algebra ofthe ideal I ⊂ R = k[x].
J bihomogeneous in the standard bigradingof k[x, y] as we are assuming that I isgenerated in fixed degree d.
We fix throughout such a set of minimalgenerators – i.e., a k-vector basis of Id.
Conferencia en Memoria de Pilar Pison Casares – p. 7/50
Rees algebras and their presentation
Let J = ker (k[x, y] ։ R(I)) denote apresentation ideal of the Rees algebra ofthe ideal I ⊂ R = k[x].
J bihomogeneous in the standard bigradingof k[x, y] as we are assuming that I isgenerated in fixed degree d.
We fix throughout such a set of minimalgenerators – i.e., a k-vector basis of Id.
Then J =⊕
(l,k) J(l,k), where J(l,k) is thek-vector spanned by the biforms of bidegree(l, k).
Conferencia en Memoria de Pilar Pison Casares – p. 7/50
Rees algebras and their presentation
Set
• J1,1 for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, 1) – call it theideal ofbilinear relations;
Conferencia en Memoria de Pilar Pison Casares – p. 8/50
Rees algebras and their presentation
Set
• J1,1 for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, 1) – call it theideal ofbilinear relations;
• J1,∗ for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, r) for all r ≥ 1 – call it theideal of semi-linear relations.
Conferencia en Memoria de Pilar Pison Casares – p. 8/50
Rees algebras and their presentation
Set
• J1,1 for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, 1) – call it theideal ofbilinear relations;
• J1,∗ for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, r) for all r ≥ 1 – call it theideal of semi-linear relations.
One has surjectiveR-algebra homomorphismsk[x, y]/J1,1 ։ k[x, y]/J1,∗ ։ R(I).
Conferencia en Memoria de Pilar Pison Casares – p. 8/50
Rees algebras and their presentation
Set
• J1,1 for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, 1) – call it theideal ofbilinear relations;
• J1,∗ for the homogeneous subideal ofJ generated bythe biforms of bidegree(1, r) for all r ≥ 1 – call it theideal of semi-linear relations.
One has surjectiveR-algebra homomorphismsk[x, y]/J1,1 ։ k[x, y]/J1,∗ ։ R(I).
Note that quite commonlyJ1,∗ = 0 and even whenJ1,∗ 6= 0, it frequently turns out thatJ1,1 = 0.
Conferencia en Memoria de Pilar Pison Casares – p. 8/50
The semilinear codimension number
Define the semilinear codimension numberof J (orperhaps of the ideal I ⊂ R = k[x] itself) as theminimum value s ≥ 1 for which htJ1,∗ = htJ1,≤s,where the latter is the subideal generated by allsemi-linear relations of bidegree (1, r) with r ≤ s.
Conferencia en Memoria de Pilar Pison Casares – p. 9/50
The semilinear codimension number
Define the semilinear codimension numberof J (orperhaps of the ideal I ⊂ R = k[x] itself) as theminimum value s ≥ 1 for which htJ1,∗ = htJ1,≤s,where the latter is the subideal generated by allsemi-linear relations of bidegree (1, r) with r ≤ s.
If htJ1,∗ = 0 we say for convenience that thesemilinear codimension number is zero.
Conferencia en Memoria de Pilar Pison Casares – p. 9/50
The semilinear codimension number
Define the semilinear codimension numberof J (orperhaps of the ideal I ⊂ R = k[x] itself) as theminimum value s ≥ 1 for which htJ1,∗ = htJ1,≤s,where the latter is the subideal generated by allsemi-linear relations of bidegree (1, r) with r ≤ s.
If htJ1,∗ = 0 we say for convenience that thesemilinear codimension number is zero.
This invariant expresses the measure of how far outthrough the successive powers of the ideal I one hasto go in order to pick a sufficient number of linearsyzygies of these powers to attain the codimensionof J1,∗.
Conferencia en Memoria de Pilar Pison Casares – p. 9/50
The semilinear codimension number
There is a related question that has been raised byWolmer Vasconcelos in connection with eliminationtheory:
Conferencia en Memoria de Pilar Pison Casares – p. 10/50
The semilinear codimension number
There is a related question that has been raised byWolmer Vasconcelos in connection with eliminationtheory:
Question Let r stand for the maximum degree of aminimal generator of the subideal ofJ generated by thebiforms of bidegree(0, s), with s ≥ 1. When isr at leastthe semilinear codimension number ofI?
Conferencia en Memoria de Pilar Pison Casares – p. 10/50
The semilinear codimension number
There is a related question that has been raised byWolmer Vasconcelos in connection with eliminationtheory:
Question Let r stand for the maximum degree of aminimal generator of the subideal ofJ generated by thebiforms of bidegree(0, s), with s ≥ 1. When isr at leastthe semilinear codimension number ofI?
As it turns this question has an affirmative answer in thecase whereI is a3-generated ideal in2-dimensionalpolynomial ring; this has recently been proved in lowdegrees byJ. Hong, W. Vasconcelos and thisspeaker and in full generality byD. Cox, J. W.Hoffman andH. Wang and, independently, byL.Buse. Conferencia en Memoria de Pilar Pison Casares – p. 10/50
SECOND PART
IDEAL THEORETICMETHODS
forRATIONAL MAPS
Conferencia en Memoria de Pilar Pison Casares – p. 11/50
Geometric setup
Fix an arbitrary field k. A rational map F : Pn
99K Pm may
be described by m+ 1 forms f = {f0, . . . , fm} of a fixed
degree ≥ 1.
Conferencia en Memoria de Pilar Pison Casares – p. 12/50
Geometric setup
Fix an arbitrary field k. A rational map F : Pn
99K Pm may
be described by m+ 1 forms f = {f0, . . . , fm} of a fixed
degree ≥ 1.
Canceling any non-trivial common factor of the forms f will
not change F ; the ideal I ⊂ k[x] generated by the resulting
forms is a uniquely defined ideal of codimension ≥ 2associated to F - called the base idealof the map F .
Conferencia en Memoria de Pilar Pison Casares – p. 12/50
Geometric setup
Fix an arbitrary field k. A rational map F : Pn
99K Pm may
be described by m+ 1 forms f = {f0, . . . , fm} of a fixed
degree ≥ 1.
Canceling any non-trivial common factor of the forms f will
not change F ; the ideal I ⊂ k[x] generated by the resulting
forms is a uniquely defined ideal of codimension ≥ 2associated to F - called the base idealof the map F .
The imageof F is the closure of the image of a non-empty
subset of Pn where F is defined.
Conferencia en Memoria de Pilar Pison Casares – p. 12/50
Geometric setup
Fix an arbitrary field k. A rational map F : Pn
99K Pm may
be described by m+ 1 forms f = {f0, . . . , fm} of a fixed
degree ≥ 1.
Canceling any non-trivial common factor of the forms f will
not change F ; the ideal I ⊂ k[x] generated by the resulting
forms is a uniquely defined ideal of codimension ≥ 2associated to F - called the base idealof the map F .
The imageof F is the closure of the image of a non-empty
subset of Pn where F is defined.
In algebraic terms, the image of F is the closed subvariety
Y ⊂ Pm whose homogeneous coordinate ring is the
k-subalgebra k[f ] ⊂ k[x], up to degree renormalization.
Conferencia en Memoria de Pilar Pison Casares – p. 12/50
Geometric setup
Recall that F is birational (onto its image Y ) if there is an
inverse rational map G : Y ⊂ Pm
99K Pn.
Conferencia en Memoria de Pilar Pison Casares – p. 13/50
Geometric setup
Recall that F is birational (onto its image Y ) if there is an
inverse rational map G : Y ⊂ Pm
99K Pn.
If G is given by g = {g0, . . . , gn} ⊂ k[y] = k[y0, . . . , ym](forms of the same degree) then the above condition is
equivalent to having the conditions:
Conferencia en Memoria de Pilar Pison Casares – p. 13/50
Geometric setup
Recall that F is birational (onto its image Y ) if there is an
inverse rational map G : Y ⊂ Pm
99K Pn.
If G is given by g = {g0, . . . , gn} ⊂ k[y] = k[y0, . . . , ym](forms of the same degree) then the above condition is
equivalent to having the conditions:
(g0(f) : . . . : gn(f)
)= (x0 : . . . : xn)
and
Conferencia en Memoria de Pilar Pison Casares – p. 13/50
Geometric setup
Recall that F is birational (onto its image Y ) if there is an
inverse rational map G : Y ⊂ Pm
99K Pn.
If G is given by g = {g0, . . . , gn} ⊂ k[y] = k[y0, . . . , ym](forms of the same degree) then the above condition is
equivalent to having the conditions:
(g0(f) : . . . : gn(f)
)= (x0 : . . . : xn)
and
(f0(g) : . . . : fm(g)
)≡ (y0 : . . . : ym) (mod I(Y ))
Conferencia en Memoria de Pilar Pison Casares – p. 13/50
Geometric setup
Recall that F is birational (onto its image Y ) if there is an
inverse rational map G : Y ⊂ Pm
99K Pn.
If G is given by g = {g0, . . . , gn} ⊂ k[y] = k[y0, . . . , ym](forms of the same degree) then the above condition is
equivalent to having the conditions:
(g0(f) : . . . : gn(f)
)= (x0 : . . . : xn)
and
(f0(g) : . . . : fm(g)
)≡ (y0 : . . . : ym) (mod I(Y ))
as tuples of homogeneous coordinates in Pmk(x) (resp. P
nK(Y ))
where K(Y ) is the field of fractions of k[y]/I(Y ).Conferencia en Memoria de Pilar Pison Casares – p. 13/50
Characteristic free criterion
These conditions can be translated into one single ring
isomorphism, namely:
Conferencia en Memoria de Pilar Pison Casares – p. 14/50
Characteristic free criterion
These conditions can be translated into one single ring
isomorphism, namely:
The identity map of k[x, y] induces a bigraded k-isomorphism
Rk[x]
((f))≃ Rk[y]
((g, I(Y ))
I(Y )
)
of Rees algebras.
Conferencia en Memoria de Pilar Pison Casares – p. 14/50
Characteristic free criterion
These conditions can be translated into one single ring
isomorphism, namely:
The identity map of k[x, y] induces a bigraded k-isomorphism
Rk[x]
((f))≃ Rk[y]
((g, I(Y ))
I(Y )
)
of Rees algebras.
This isomorphism in turn yields a module theoretic criterions
as follows.
Conferencia en Memoria de Pilar Pison Casares – p. 14/50
Characteristic free criterion
These conditions can be translated into one single ring
isomorphism, namely:
The identity map of k[x, y] induces a bigraded k-isomorphism
Rk[x]
((f))≃ Rk[y]
((g, I(Y ))
I(Y )
)
of Rees algebras.
This isomorphism in turn yields a module theoretic criterions
as follows.
Recap the previous notation: J ⊂ k[x, y] is the presentation
ideal of Rk[x](f) and J1,∗ is the subideal generated by all
biforms of bidegree (1, r), with r ≥ 1Conferencia en Memoria de Pilar Pison Casares – p. 14/50
Characteristic free criterion
Consider a minimal set of biforms generating J1,∗ and let ψdenote the Jacobian matrix of these biforms with respect to
the x-variables.
Conferencia en Memoria de Pilar Pison Casares – p. 15/50
Characteristic free criterion
Consider a minimal set of biforms generating J1,∗ and let ψdenote the Jacobian matrix of these biforms with respect to
the x-variables.
Clearly, ψ is a matrix with n+ 1 columns and entries in k[y]and it is graded since its rows are homogeneous vectors in
k[y].
Conferencia en Memoria de Pilar Pison Casares – p. 15/50
Characteristic free criterion
Consider a minimal set of biforms generating J1,∗ and let ψdenote the Jacobian matrix of these biforms with respect to
the x-variables.
Clearly, ψ is a matrix with n+ 1 columns and entries in k[y]and it is graded since its rows are homogeneous vectors in
k[y].
Theorem. Let S = k[y]/I(Y ) ≃ k[f ], the homogeneous
coordinate ring of the image Y ⊂ Pm of the rational map
F : Pn
99K Pm defined by f . Assume that dimS = n+ 1.
The following condition are equivalent:
Conferencia en Memoria de Pilar Pison Casares – p. 15/50
Characteristic free criterion
Consider a minimal set of biforms generating J1,∗ and let ψdenote the Jacobian matrix of these biforms with respect to
the x-variables.
Clearly, ψ is a matrix with n+ 1 columns and entries in k[y]and it is graded since its rows are homogeneous vectors in
k[y].
Theorem. Let S = k[y]/I(Y ) ≃ k[f ], the homogeneous
coordinate ring of the image Y ⊂ Pm of the rational map
F : Pn
99K Pm defined by f . Assume that dimS = n+ 1.
The following condition are equivalent:
(i) F is birational onto Y
Conferencia en Memoria de Pilar Pison Casares – p. 15/50
Characteristic free criterion
Consider a minimal set of biforms generating J1,∗ and let ψdenote the Jacobian matrix of these biforms with respect to
the x-variables.
Clearly, ψ is a matrix with n+ 1 columns and entries in k[y]and it is graded since its rows are homogeneous vectors in
k[y].
Theorem. Let S = k[y]/I(Y ) ≃ k[f ], the homogeneous
coordinate ring of the image Y ⊂ Pm of the rational map
F : Pn
99K Pm defined by f . Assume that dimS = n+ 1.
The following condition are equivalent:
(i) F is birational onto Y(ii) rankS(ψ) = n and Im((ψ)t) = Im(ψ)∗, where t denotes
transpose and ∗ denotes S-dual.Conferencia en Memoria de Pilar Pison Casares – p. 15/50
Characteristic free criterion
Supplement. Moreover, when condition (ii) holds, the
coordinates of any homogeneous syzygy of ψ over S define the
inverse map to F .
Conferencia en Memoria de Pilar Pison Casares – p. 16/50
Characteristic free criterion
Supplement. Moreover, when condition (ii) holds, the
coordinates of any homogeneous syzygy of ψ over S define the
inverse map to F .
Here is a more tangible special case:
Conferencia en Memoria de Pilar Pison Casares – p. 16/50
Characteristic free criterion
Supplement. Moreover, when condition (ii) holds, the
coordinates of any homogeneous syzygy of ψ over S define the
inverse map to F .
Here is a more tangible special case:
Corollary. Assumptions as above. Suppose thatrankS(ψ) = n, that the ideal of n-minors of ψcontains a regular sequence of length ≥ 2 over Sand that the cokernel of the transpose ψt over S istorsionfree. Then F is birational onto Y .
Conferencia en Memoria de Pilar Pison Casares – p. 16/50
The role of the linear syzygies
We now deal with the linear syzygies of the baseideal more closely in order to establish theirgeometric meaning.
Conferencia en Memoria de Pilar Pison Casares – p. 17/50
The role of the linear syzygies
We now deal with the linear syzygies of the baseideal more closely in order to establish theirgeometric meaning.
Lemma (Assume k is algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Conferencia en Memoria de Pilar Pison Casares – p. 17/50
The role of the linear syzygies
We now deal with the linear syzygies of the baseideal more closely in order to establish theirgeometric meaning.
Lemma (Assume k is algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Set L = J1,1 + I(Y ) ⊂ k[x, y], where J1,1 is the
previously introduced ideal of bilinear relations.
Conferencia en Memoria de Pilar Pison Casares – p. 17/50
The role of the linear syzygies
We now deal with the linear syzygies of the baseideal more closely in order to establish theirgeometric meaning.
Lemma (Assume k is algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Set L = J1,1 + I(Y ) ⊂ k[x, y], where J1,1 is the
previously introduced ideal of bilinear relations.
Then the fiber of BiProj(k[x, y]/L) → Y over any
closed point p ∈ Y is a “vertical” linear subspaceL× {p} ⊂ P
n × {p}.Conferencia en Memoria de Pilar Pison Casares – p. 17/50
The role of the linear syzygies
We note that, quite generally, BiProj(R(I)) is aclosed subvariety of BiProj(k[x, y]/L), where
I ⊂ k[x] denotes the base ideal of F .
Conferencia en Memoria de Pilar Pison Casares – p. 18/50
The role of the linear syzygies
We note that, quite generally, BiProj(R(I)) is aclosed subvariety of BiProj(k[x, y]/L), where
I ⊂ k[x] denotes the base ideal of F .
Proposition (k algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Conferencia en Memoria de Pilar Pison Casares – p. 18/50
The role of the linear syzygies
We note that, quite generally, BiProj(R(I)) is aclosed subvariety of BiProj(k[x, y]/L), where
I ⊂ k[x] denotes the base ideal of F .
Proposition (k algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Suppose that BiProj(R(I)) is an irreduciblecomponent of BiProj(k[x, y]/L).
Conferencia en Memoria de Pilar Pison Casares – p. 18/50
The role of the linear syzygies
We note that, quite generally, BiProj(R(I)) is aclosed subvariety of BiProj(k[x, y]/L), where
I ⊂ k[x] denotes the base ideal of F .
Proposition (k algebraically closed)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Suppose that BiProj(R(I)) is an irreduciblecomponent of BiProj(k[x, y]/L).
Then the fiber of BiProj(R(I)) → Y over a generalclosed point p ∈ Y is again a “vertical” linearsubspace L× {p} ⊂ P
n × {p}.Conferencia en Memoria de Pilar Pison Casares – p. 18/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Corollary (k algebraically closed, char(k) = 0)Let F : Pn
99K Pm be a rational map with imageY ⊂ P
m.
Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Corollary (k algebraically closed, char(k) = 0)Let F : Pn
99K Pm be a rational map with imageY ⊂ P
m.
With the previous notation suppose that:
Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Corollary (k algebraically closed, char(k) = 0)Let F : Pn
99K Pm be a rational map with imageY ⊂ P
m.
With the previous notation suppose that:
• BiProj(R(I)) is an irreducible component ofBiProj(k[x, y]/L).
Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Corollary (k algebraically closed, char(k) = 0)Let F : Pn
99K Pm be a rational map with imageY ⊂ P
m.
With the previous notation suppose that:
• BiProj(R(I)) is an irreducible component ofBiProj(k[x, y]/L).
• dimY = n
Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
The interesting consequences of this method forbirationality are, unfortunately, not characteristicfree.
Corollary (k algebraically closed, char(k) = 0)Let F : Pn
99K Pm be a rational map with imageY ⊂ P
m.
With the previous notation suppose that:
• BiProj(R(I)) is an irreducible component ofBiProj(k[x, y]/L).
• dimY = n
Then F is birational onto Y .Conferencia en Memoria de Pilar Pison Casares – p. 19/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Corollary (k algebraically closed, char(k) = 0)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Corollary (k algebraically closed, char(k) = 0)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
In an earlier terminology suppose that:
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Corollary (k algebraically closed, char(k) = 0)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
In an earlier terminology suppose that:
• The presentation matrix ϕ of the base ideal hasmaximal linear rank.
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Corollary (k algebraically closed, char(k) = 0)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
In an earlier terminology suppose that:
• The presentation matrix ϕ of the base ideal hasmaximal linear rank.
• dimY = n
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Checking whether a prime is minimal is not sohorrend. However, to stay strictly effective, thefollowing version may be used:
Corollary (k algebraically closed, char(k) = 0)Let F : P
n99K P
m be a rational map with imageY ⊂ P
m.
In an earlier terminology suppose that:
• The presentation matrix ϕ of the base ideal hasmaximal linear rank.
• dimY = n
Then F is birational onto Y .
Conferencia en Memoria de Pilar Pison Casares – p. 20/50
The role of the linear syzygies
Remark The last corollary follows from the precedent by
showing that whenever rank(ϕ1) = rank(ϕ) then:
Conferencia en Memoria de Pilar Pison Casares – p. 21/50
The role of the linear syzygies
Remark The last corollary follows from the precedent by
showing that whenever rank(ϕ1) = rank(ϕ) then:
• There is a natural isomorphism of Rees algebrasR(E) ≃ R(I), where E = coker(ϕ1);
Conferencia en Memoria de Pilar Pison Casares – p. 21/50
The role of the linear syzygies
Remark The last corollary follows from the precedent by
showing that whenever rank(ϕ1) = rank(ϕ) then:
• There is a natural isomorphism of Rees algebrasR(E) ≃ R(I), where E = coker(ϕ1);
• There is an isomorphism S(E) ≃ k[x, y]/J1,1,
hence ker (k[x, y]/J1,1 → R(I)) is the k[x]-torsion of
R(E);
Conferencia en Memoria de Pilar Pison Casares – p. 21/50
The role of the linear syzygies
Remark The last corollary follows from the precedent by
showing that whenever rank(ϕ1) = rank(ϕ) then:
• There is a natural isomorphism of Rees algebrasR(E) ≃ R(I), where E = coker(ϕ1);
• There is an isomorphism S(E) ≃ k[x, y]/J1,1,
hence ker (k[x, y]/J1,1 → R(I)) is the k[x]-torsion of
R(E);
• The k[x]-torsion of R(E) is a minimal prime ofS(E).
Conferencia en Memoria de Pilar Pison Casares – p. 21/50
The role of the linear syzygies
Remark The last corollary follows from the precedent by
showing that whenever rank(ϕ1) = rank(ϕ) then:
• There is a natural isomorphism of Rees algebrasR(E) ≃ R(I), where E = coker(ϕ1);
• There is an isomorphism S(E) ≃ k[x, y]/J1,1,
hence ker (k[x, y]/J1,1 → R(I)) is the k[x]-torsion of
R(E);
• The k[x]-torsion of R(E) is a minimal prime ofS(E).
• It follows that ker (k[x, y]/J1,1 → R(I)) is a
minimal prime of k[x, y]/L as well.Conferencia en Memoria de Pilar Pison Casares – p. 21/50
THIRD PART
Cremona transformations
Conferencia en Memoria de Pilar Pison Casares – p. 22/50
The general algebraic criterion
Definition. A Cremona transformation of Pn is a
birational map of Pn onto itself. The designation honors
Luigi Cremona, winner of the Steiner prize - not the
homonymous city in Italy.
Conferencia en Memoria de Pilar Pison Casares – p. 23/50
The general algebraic criterion
Definition. A Cremona transformation of Pn is a
birational map of Pn onto itself. The designation honors
Luigi Cremona, winner of the Steiner prize - not the
homonymous city in Italy.
We may read out the earlier criterion in this case in the
following way, where hd(M) denotes the homological
dimension of a module M :
Conferencia en Memoria de Pilar Pison Casares – p. 23/50
The general algebraic criterion
Definition. A Cremona transformation of Pn is a
birational map of Pn onto itself. The designation honors
Luigi Cremona, winner of the Steiner prize - not the
homonymous city in Italy.
We may read out the earlier criterion in this case in the
following way, where hd(M) denotes the homological
dimension of a module M :
F : Pn
99K Pn is a Cremona transformation if and only if f
is algebraically independent over k, the matrix ψ has rank nand hd(coker(ψ)t)P ) ≤ ht (P ) − 1 for every nonzero prime
ideal P ⊂ k[x].
Conferencia en Memoria de Pilar Pison Casares – p. 23/50
Syzygetic criteria
Let F : Pn
99K Pn be a rational map defined by forms
f ⊂ k[x]. Denote by θ(f) the Jacobian matrix of f . As an
immediate application of the previous results one has
conditions in order that F be a Cremona map solely in terms
of the two main matrices attached to f :
Conferencia en Memoria de Pilar Pison Casares – p. 24/50
Syzygetic criteria
Let F : Pn
99K Pn be a rational map defined by forms
f ⊂ k[x]. Denote by θ(f) the Jacobian matrix of f . As an
immediate application of the previous results one has
conditions in order that F be a Cremona map solely in terms
of the two main matrices attached to f :
Proposition (k algebraically closed, char(k) = 0) If
rank(θ(f)) = n + 1 and rank(ϕ1) = n then F is a
Cremona map.
Conferencia en Memoria de Pilar Pison Casares – p. 24/50
Syzygetic criteria
Let F : Pn
99K Pn be a rational map defined by forms
f ⊂ k[x]. Denote by θ(f) the Jacobian matrix of f . As an
immediate application of the previous results one has
conditions in order that F be a Cremona map solely in terms
of the two main matrices attached to f :
Proposition (k algebraically closed, char(k) = 0) If
rank(θ(f)) = n + 1 and rank(ϕ1) = n then F is a
Cremona map.
Observe that the role of the two matrices is asymmetrical: the
condition on θ(f) is necessary, while the one on ϕ is
(partially) sufficient.
Conferencia en Memoria de Pilar Pison Casares – p. 24/50
Syzygetic criteria
Under a condition stronger than algebraic independence, the
condition on the syzygies of the base ideal is just right in order
that F : Pn
99K Pn be a Cremona map.
Conferencia en Memoria de Pilar Pison Casares – p. 25/50
Syzygetic criteria
Under a condition stronger than algebraic independence, the
condition on the syzygies of the base ideal is just right in order
that F : Pn
99K Pn be a Cremona map.
Proposition (k algebraically closed, char(k) = 0)
Suppose that the ideal (f) ⊂ k[x] is of linear type. Then
F : Pn
99K Pn is a Cremona map if and only if the syzygies
of f have maximal linear rank.
Conferencia en Memoria de Pilar Pison Casares – p. 25/50
Syzygetic criteria
Under a condition stronger than algebraic independence, the
condition on the syzygies of the base ideal is just right in order
that F : Pn
99K Pn be a Cremona map.
Proposition (k algebraically closed, char(k) = 0)
Suppose that the ideal (f) ⊂ k[x] is of linear type. Then
F : Pn
99K Pn is a Cremona map if and only if the syzygies
of f have maximal linear rank.
Supplement Moreover, if Im(ϕ1) ⊂ k[x]n+1 is a free
submodule of rank n then the base ideal of the inverse map
F−1 is a codimension two perfect ideal generated by forms of
degree n.
Conferencia en Memoria de Pilar Pison Casares – p. 25/50
Examples, I
1. – For an integer m ≥ 1 let X (respectively, X̃ ) stand for
the m×m generic (respectively, generic symmetric) matrix.
Conferencia en Memoria de Pilar Pison Casares – p. 26/50
Examples, I
1. – For an integer m ≥ 1 let X (respectively, X̃ ) stand for
the m×m generic (respectively, generic symmetric) matrix.
Let f (respectively, f̃ ) denote the submaximal minors of X
(respectively, of X̃ ).
Conferencia en Memoria de Pilar Pison Casares – p. 26/50
Examples, I
1. – For an integer m ≥ 1 let X (respectively, X̃ ) stand for
the m×m generic (respectively, generic symmetric) matrix.
Let f (respectively, f̃ ) denote the submaximal minors of X
(respectively, of X̃ ).
The rational map F : Pm2−1
99K Pm2−1 (respectively,
F̃ : P(m+1
2 )−199K P(m+1
2 )−1) defined by f (respectively, by
f̃ ) is a Cremona map.
Conferencia en Memoria de Pilar Pison Casares – p. 26/50
Examples, I
1. – For an integer m ≥ 1 let X (respectively, X̃ ) stand for
the m×m generic (respectively, generic symmetric) matrix.
Let f (respectively, f̃ ) denote the submaximal minors of X
(respectively, of X̃ ).
The rational map F : Pm2−1
99K Pm2−1 (respectively,
F̃ : P(m+1
2 )−199K P(m+1
2 )−1) defined by f (respectively, by
f̃ ) is a Cremona map.
We note that in both cases the partial derivatives of the
respective determinant coincide with the (signed) cofactors of
the matrix; these are, therefore, examples of polar Cremona
maps, to be seen in the next part.Conferencia en Memoria de Pilar Pison Casares – p. 26/50
Examples, II
2. – Consider the generic 3 × 3 Hankel matrix
Conferencia en Memoria de Pilar Pison Casares – p. 27/50
Examples, II
2. – Consider the generic 3 × 3 Hankel matrix
H =
x0 x1 x2
x1 x2 x3
x2 x3 x4
Conferencia en Memoria de Pilar Pison Casares – p. 27/50
Examples, II
2. – Consider the generic 3 × 3 Hankel matrix
H =
x0 x1 x2
x1 x2 x3
x2 x3 x4
Let f stand for the partial derivatives of detH and let
F : P499K P
4 denote the rational map defined by f .
Conferencia en Memoria de Pilar Pison Casares – p. 27/50
Examples, II
2. – Consider the generic 3 × 3 Hankel matrix
H =
x0 x1 x2
x1 x2 x3
x2 x3 x4
Let f stand for the partial derivatives of detH and let
F : P499K P
4 denote the rational map defined by f .
Note that the cofactors define now a rational map
G : P499K P
5 which is birational onto its image, the
Grassmannian quadric G(1, 3) ⊂ P5. This is either classical
or a consequence of the minors generating a linearly presented
ideal.
Conferencia en Memoria de Pilar Pison Casares – p. 27/50
Examples, II
On the other hand, F is the composition of G and the linear
projection P : P599K P
4 from the point
(0 : 0 : 1/3 : −1 : 0 : 0) ∈ G(1, 3).
Conferencia en Memoria de Pilar Pison Casares – p. 28/50
Examples, II
On the other hand, F is the composition of G and the linear
projection P : P599K P
4 from the point
(0 : 0 : 1/3 : −1 : 0 : 0) ∈ G(1, 3).
Moreover, the restriction of P to the quadric is generically a
double covering.
Conferencia en Memoria de Pilar Pison Casares – p. 28/50
Examples, II
On the other hand, F is the composition of G and the linear
projection P : P599K P
4 from the point
(0 : 0 : 1/3 : −1 : 0 : 0) ∈ G(1, 3).
Moreover, the restriction of P to the quadric is generically a
double covering.
It follows that F is not a Cremona map.
Conferencia en Memoria de Pilar Pison Casares – p. 28/50
Examples, II
On the other hand, F is the composition of G and the linear
projection P : P599K P
4 from the point
(0 : 0 : 1/3 : −1 : 0 : 0) ∈ G(1, 3).
Moreover, the restriction of P to the quadric is generically a
double covering.
It follows that F is not a Cremona map.
Still in this case the partial derivatives f generate an ideal of
linear type. Thus, by the previous criterion, it must be the
case that the syzygies of this ideal are not of maximal linear
rank. And indeed, this can be laboriously proved by
specializing from the generic symmetric case or else by a
computer calculation.
Conferencia en Memoria de Pilar Pison Casares – p. 28/50
FOURTH PART
Polar Cremonatransformations
Conferencia en Memoria de Pilar Pison Casares – p. 29/50
Generalities
Definitions. A homogeneous polynomial f ∈ k[x]determines a polar mapPf : P
n99K P
n defined by itspartial derivatives.
Conferencia en Memoria de Pilar Pison Casares – p. 30/50
Generalities
Definitions. A homogeneous polynomial f ∈ k[x]determines a polar mapPf : P
n99K P
n defined by itspartial derivatives.
One says that f is homaloidalif Pf is a Cremonatransformation of P
n, in which case one refers to Pf
as being a polar Cremona map.
Conferencia en Memoria de Pilar Pison Casares – p. 30/50
Generalities
Definitions. A homogeneous polynomial f ∈ k[x]determines a polar mapPf : P
n99K P
n defined by itspartial derivatives.
One says that f is homaloidalif Pf is a Cremonatransformation of P
n, in which case one refers to Pf
as being a polar Cremona map.For n = 2 the classification was given by I.Dolgachevand the possible degrees are ≤ 3 = n+ 1
Conferencia en Memoria de Pilar Pison Casares – p. 30/50
Generalities
Definitions. A homogeneous polynomial f ∈ k[x]determines a polar mapPf : P
n99K P
n defined by itspartial derivatives.
One says that f is homaloidalif Pf is a Cremonatransformation of P
n, in which case one refers to Pf
as being a polar Cremona map.For n = 2 the classification was given by I.Dolgachevand the possible degrees are ≤ 3 = n+ 1
For n ≥ 3 any degree is attainable by irreduciblehomaloidal polynomials (work of C. Ciliberto, F.Russoand this speaker)
Conferencia en Memoria de Pilar Pison Casares – p. 30/50
The class of sub–Hankel matrices
We start with a class of homaloidal polynomials that admit an
explicit description and are, moreover, absolutely irreducible.
Conferencia en Memoria de Pilar Pison Casares – p. 31/50
The class of sub–Hankel matrices
We start with a class of homaloidal polynomials that admit an
explicit description and are, moreover, absolutely irreducible.
The generic sub–Hankel matrix of orderr on the variables
x0, . . . , xn is the matrix
Conferencia en Memoria de Pilar Pison Casares – p. 31/50
The class of sub–Hankel matrices
We start with a class of homaloidal polynomials that admit an
explicit description and are, moreover, absolutely irreducible.
The generic sub–Hankel matrix of orderr on the variables
x0, . . . , xn is the matrix
x0 x1 x2 ... xn−2 xn−1
x1 x2 x3 ... xn−1 xn
x2 x3 x4 ... xn 0
. . . ... . .
. . . ... . .
. . . ... . .
xn−2 xn−1 xn ... 0 0
xn−1 xn 0 ... 0 0
Conferencia en Memoria de Pilar Pison Casares – p. 31/50
The class of sub–Hankel matrices
Theorem. Let n ≥ 2. Let f stand for thedeterminant of the sub–Hankel matrix and setJ = (∂f/∂x0, . . . , ∂f/∂xn). Then:
Conferencia en Memoria de Pilar Pison Casares – p. 32/50
The class of sub–Hankel matrices
Theorem. Let n ≥ 2. Let f stand for thedeterminant of the sub–Hankel matrix and setJ = (∂f/∂x0, . . . , ∂f/∂xn). Then:
• For every value of i in the range 1 ≤ i ≤ n− 1, thepartial derivatives ∂f/∂x0, . . . , ∂f/∂xi divided bytheir g.c.d. define a Cremona transformation of P
i;
Conferencia en Memoria de Pilar Pison Casares – p. 32/50
The class of sub–Hankel matrices
Theorem. Let n ≥ 2. Let f stand for thedeterminant of the sub–Hankel matrix and setJ = (∂f/∂x0, . . . , ∂f/∂xn). Then:
• For every value of i in the range 1 ≤ i ≤ n− 1, thepartial derivatives ∂f/∂x0, . . . , ∂f/∂xi divided bytheir g.c.d. define a Cremona transformation of P
i;
• In addition, both base loci of this Cremonatransformation and of its inverse map are defined bycodimension two perfect ideals of linear type,generated in degree i;
Conferencia en Memoria de Pilar Pison Casares – p. 32/50
The class of sub–Hankel matrices
Theorem. Let n ≥ 2. Let f stand for thedeterminant of the sub–Hankel matrix and setJ = (∂f/∂x0, . . . , ∂f/∂xn). Then:
• For every value of i in the range 1 ≤ i ≤ n− 1, thepartial derivatives ∂f/∂x0, . . . , ∂f/∂xi divided bytheir g.c.d. define a Cremona transformation of P
i;
• In addition, both base loci of this Cremonatransformation and of its inverse map are defined bycodimension two perfect ideals of linear type,generated in degree i;
• The Hessian of f has the form h(f) = c x(r+1)(r−2)r ,
for some c ∈ k, c 6= 0Conferencia en Memoria de Pilar Pison Casares – p. 32/50
The class of sub–Hankel matrices
Moreover:
Conferencia en Memoria de Pilar Pison Casares – p. 33/50
The class of sub–Hankel matrices
Moreover:
• The syzygies of J have maximal linear rank;
Conferencia en Memoria de Pilar Pison Casares – p. 33/50
The class of sub–Hankel matrices
Moreover:
• The syzygies of J have maximal linear rank;
• As a consequence of previous criteria, if the fieldhas characteristic zero then f is a homaloidalpolynomial.
Conferencia en Memoria de Pilar Pison Casares – p. 33/50
The class of sub–Hankel matrices
Moreover:
• The syzygies of J have maximal linear rank;
• As a consequence of previous criteria, if the fieldhas characteristic zero then f is a homaloidalpolynomial.
Comment. The proof of the first of these resultsrelies on a mix of fairly technical non-trivialinductive formulas involving partial derivatives. Itwould be curious to have a geometric approachshowing that the corresponding polar map isCremona.
Conferencia en Memoria de Pilar Pison Casares – p. 33/50
The class of sub–Hankel matrices
Conferencia en Memoria de Pilar Pison Casares – p. 34/50
The class of sub–Hankel matrices
• The degree of a sub–Hankel determinant is the dimension of
the ambient space, while many of the known classical
examples of homaloidal polynomials have a small degree as
confronted with the number of variables;
Conferencia en Memoria de Pilar Pison Casares – p. 34/50
The class of sub–Hankel matrices
• The degree of a sub–Hankel determinant is the dimension of
the ambient space, while many of the known classical
examples of homaloidal polynomials have a small degree as
confronted with the number of variables;
• The singular locus of a sub–Hankel hypersurface is a
multiple structure over the codimension 2 linear subspace
xr−1 = xr = 0 off the codimension 3 subspace
xr−2 = xr−1 = xr = 0, while the generators of any Ji define
a generalized de Jonquieres transformation;
Conferencia en Memoria de Pilar Pison Casares – p. 34/50
The class of sub–Hankel matrices
• The degree of a sub–Hankel determinant is the dimension of
the ambient space, while many of the known classical
examples of homaloidal polynomials have a small degree as
confronted with the number of variables;
• The singular locus of a sub–Hankel hypersurface is a
multiple structure over the codimension 2 linear subspace
xr−1 = xr = 0 off the codimension 3 subspace
xr−2 = xr−1 = xr = 0, while the generators of any Ji define
a generalized de Jonquieres transformation;
• The Hessian of a sub–Hankel determinant is essentially a
power of a linear form. Ciliberto has asked whether it is
possible to characterize the class of homaloidal polynomials
whose Hessian is of this form.
Conferencia en Memoria de Pilar Pison Casares – p. 34/50
Homology of a sub–Hankel matrix
In this slide we deal with further algebraic properties of
sub–Hankel matrices. The main result of this part is as follows.
Conferencia en Memoria de Pilar Pison Casares – p. 35/50
Homology of a sub–Hankel matrix
In this slide we deal with further algebraic properties of
sub–Hankel matrices. The main result of this part is as follows.
Theorem Let J ⊂ R = k[x] denote the gradient ideal of
the sub–Hankel determinant f as above. Then
Conferencia en Memoria de Pilar Pison Casares – p. 35/50
Homology of a sub–Hankel matrix
In this slide we deal with further algebraic properties of
sub–Hankel matrices. The main result of this part is as follows.
Theorem Let J ⊂ R = k[x] denote the gradient ideal of
the sub–Hankel determinant f as above. Then
• The minimal graded free resolution of J over R is the
mapping cone of a map between the minimal graded free
resolution of Jn−1 and the graded free resolution of the shifted
complete intersection R
(xn,xn−1
n−1)(−(n− 1));
Conferencia en Memoria de Pilar Pison Casares – p. 35/50
Homology of a sub–Hankel matrix
In this slide we deal with further algebraic properties of
sub–Hankel matrices. The main result of this part is as follows.
Theorem Let J ⊂ R = k[x] denote the gradient ideal of
the sub–Hankel determinant f as above. Then
• The minimal graded free resolution of J over R is the
mapping cone of a map between the minimal graded free
resolution of Jn−1 and the graded free resolution of the shifted
complete intersection R
(xn,xn−1
n−1)(−(n− 1));
• J is a (codimension two non-perfect) ideal of linear type.
Conferencia en Memoria de Pilar Pison Casares – p. 35/50
Shifted sub–Hankel matrices
As it turns, the shifted versions of a sub–Hankel matrix
produce irreducible homaloidal determinants as well. Namely,
for a fixed integer r, 1 ≤ r ≤ n− 1, look at the following
(r + 1) × (r + 1) matrix
Conferencia en Memoria de Pilar Pison Casares – p. 36/50
Shifted sub–Hankel matrices
As it turns, the shifted versions of a sub–Hankel matrix
produce irreducible homaloidal determinants as well. Namely,
for a fixed integer r, 1 ≤ r ≤ n− 1, look at the following
(r + 1) × (r + 1) matrix
x0 x1 x2 . . . xr−1 xr
xn−r xn−r+1 xn−r+2 . . . xn−1 xn
xn−r+1 xn−r+2 xn−r+3 . . . xn 0...
......
......
...
xn−2 xn−1 xn . . . 0 0
xn−1 xn 0 . . . 0 0
Conferencia en Memoria de Pilar Pison Casares – p. 36/50
Shifted sub–Hankel matrices
Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Shifted sub–Hankel matrices
As before, we denote by J the gradient ideal of its
determinant. The first case (r = 2, n = 4) does not present
any new phenomena as the syzygies of J have maximal linear
rank and the Hessian of g is of the form cx54, with a nonzero
c ∈ k;
Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Shifted sub–Hankel matrices
As before, we denote by J the gradient ideal of its
determinant. The first case (r = 2, n = 4) does not present
any new phenomena as the syzygies of J have maximal linear
rank and the Hessian of g is of the form cx54, with a nonzero
c ∈ k;
Next relevant case (r = 3, n = 5) has some definite
anomalies:
Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Shifted sub–Hankel matrices
As before, we denote by J the gradient ideal of its
determinant. The first case (r = 2, n = 4) does not present
any new phenomena as the syzygies of J have maximal linear
rank and the Hessian of g is of the form cx54, with a nonzero
c ∈ k;
Next relevant case (r = 3, n = 5) has some definite
anomalies:
• The linear rank of the syzygies of g is one less than the
maximal value;
Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Shifted sub–Hankel matrices
As before, we denote by J the gradient ideal of its
determinant. The first case (r = 2, n = 4) does not present
any new phenomena as the syzygies of J have maximal linear
rank and the Hessian of g is of the form cx54, with a nonzero
c ∈ k;
Next relevant case (r = 3, n = 5) has some definite
anomalies:
• The linear rank of the syzygies of g is one less than the
maximal value;
• g is nevertheless homaloidal and the inverse to its polar map
Pg has a perfect codimension two base ideal;
Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Shifted sub–Hankel matrices
As before, we denote by J the gradient ideal of its
determinant. The first case (r = 2, n = 4) does not present
any new phenomena as the syzygies of J have maximal linear
rank and the Hessian of g is of the form cx54, with a nonzero
c ∈ k;
Next relevant case (r = 3, n = 5) has some definite
anomalies:
• The linear rank of the syzygies of g is one less than the
maximal value;
• g is nevertheless homaloidal and the inverse to its polar map
Pg has a perfect codimension two base ideal;
• Neither the base ideal of Pg nor its inverse base ideal is of
linear type. Conferencia en Memoria de Pilar Pison Casares – p. 37/50
Sporadic examples, I. The circulant
There is yet another way in which the “schiebung” principle
can be applied and that is as “the thickened circulant”. We
will comprise only the case of the circulant itself:
Conferencia en Memoria de Pilar Pison Casares – p. 38/50
Sporadic examples, I. The circulant
There is yet another way in which the “schiebung” principle
can be applied and that is as “the thickened circulant”. We
will comprise only the case of the circulant itself:
x0 x1 . . . xn−1 xn
x1 x2 . . . xn x0
x2 x3 . . . x0 x1...
xn−1 xn . . . xn−3 xn−2
xn x0 . . . xn−2 xn−1
Conferencia en Memoria de Pilar Pison Casares – p. 38/50
Sporadic examples, I. The circulant
Here is the expected structural result on the homaloidal
nature of the circulant:
Conferencia en Memoria de Pilar Pison Casares – p. 39/50
Sporadic examples, I. The circulant
Here is the expected structural result on the homaloidal
nature of the circulant:
• The partial derivatives of the circulant determinant are the
(signed) cofactors of (any) row (or column) up to a constant
multiplier;
Conferencia en Memoria de Pilar Pison Casares – p. 39/50
Sporadic examples, I. The circulant
Here is the expected structural result on the homaloidal
nature of the circulant:
• The partial derivatives of the circulant determinant are the
(signed) cofactors of (any) row (or column) up to a constant
multiplier;
• The partial derivatives of the circulant determinant generate
a codimension 2 linearly presented perfect ideal;
Conferencia en Memoria de Pilar Pison Casares – p. 39/50
Sporadic examples, I. The circulant
Here is the expected structural result on the homaloidal
nature of the circulant:
• The partial derivatives of the circulant determinant are the
(signed) cofactors of (any) row (or column) up to a constant
multiplier;
• The partial derivatives of the circulant determinant generate
a codimension 2 linearly presented perfect ideal;
• The circulant determinant is a squarefree homaloidal
polynomial.
Conferencia en Memoria de Pilar Pison Casares – p. 39/50
Sporadic examples, I. The circulant
Here is the expected structural result on the homaloidal
nature of the circulant:
• The partial derivatives of the circulant determinant are the
(signed) cofactors of (any) row (or column) up to a constant
multiplier;
• The partial derivatives of the circulant determinant generate
a codimension 2 linearly presented perfect ideal;
• The circulant determinant is a squarefree homaloidal
polynomial.
Remark The first of these says that the circulant
determinant is a free divisor in the theory of K. Saito – it is
actually a Koszul free divisor in the terminology of the
Sevilla school. Conferencia en Memoria de Pilar Pison Casares – p. 39/50
Sporadic examples, I. The circulant
Alas, the circulant determinant factors completelyover C:
Conferencia en Memoria de Pilar Pison Casares – p. 40/50
Sporadic examples, I. The circulant
Alas, the circulant determinant factors completelyover C:
Assume that k contains a primitive (n+ 1)th root ofunity ζ and let σ denote the permutation of theroots {1, ζ, ζ2, . . . , ζn} such that σ(1) = 1, σ(ζn) = ζand σ(ζ i) = ζ i+1, for 1 ≤ i ≤ n− 1. Then thecirculant determinant factors as
Conferencia en Memoria de Pilar Pison Casares – p. 40/50
Sporadic examples, I. The circulant
Alas, the circulant determinant factors completelyover C:
Assume that k contains a primitive (n+ 1)th root ofunity ζ and let σ denote the permutation of theroots {1, ζ, ζ2, . . . , ζn} such that σ(1) = 1, σ(ζn) = ζand σ(ζ i) = ζ i+1, for 1 ≤ i ≤ n− 1. Then thecirculant determinant factors as
(x0 + · · · + xn)n−1∏
j=0
(n∑
i=0
σj(ζ i)xi
)
Conferencia en Memoria de Pilar Pison Casares – p. 40/50
Sporadic examples, I. The circulant
Conferencia en Memoria de Pilar Pison Casares – p. 41/50
Sporadic examples, I. The circulant
Since the factors are all distinct, it is just a Steinerdeterminant up to a collineation.
Conferencia en Memoria de Pilar Pison Casares – p. 41/50
Sporadic examples, I. The circulant
Since the factors are all distinct, it is just a Steinerdeterminant up to a collineation.
However, over R, for example, the residual factorofx0 + · · · + xn is irreducible and can be seen to betotally Hessian, hence is homaloidal. Our presentknowledge of the theory, as vindicated in this talk,is not enough to decide whether the inverse polarmap is defined over R.
Conferencia en Memoria de Pilar Pison Casares – p. 41/50
Sporadic examples, I. The circulant
Since the factors are all distinct, it is just a Steinerdeterminant up to a collineation.
However, over R, for example, the residual factorofx0 + · · · + xn is irreducible and can be seen to betotally Hessian, hence is homaloidal. Our presentknowledge of the theory, as vindicated in this talk,is not enough to decide whether the inverse polarmap is defined over R.Thus, one may wonder if there is a genuine theoryof “rational” homaloidal polynomials; if this is case,one would expect the classification results to bemuch harder.
Conferencia en Memoria de Pilar Pison Casares – p. 41/50
Sporadic examples, II. Dual of ratio-nal normal curve
The following instance is a lot speculative; it hasbeen verified up to n ≤ 5.
Conferencia en Memoria de Pilar Pison Casares – p. 42/50
Sporadic examples, II. Dual of ratio-nal normal curve
The following instance is a lot speculative; it hasbeen verified up to n ≤ 5.
The rational normal curve in Pn is known to be
non-defective. Let fn stand for its dualhypersurface.
Conferencia en Memoria de Pilar Pison Casares – p. 42/50
Sporadic examples, II. Dual of ratio-nal normal curve
The following instance is a lot speculative; it hasbeen verified up to n ≤ 5.
The rational normal curve in Pn is known to be
non-defective. Let fn stand for its dualhypersurface.
• The partial derivatives of fn generate acodimension two perfect ideal of linear type;
Conferencia en Memoria de Pilar Pison Casares – p. 42/50
Sporadic examples, II. Dual of ratio-nal normal curve
The following instance is a lot speculative; it hasbeen verified up to n ≤ 5.
The rational normal curve in Pn is known to be
non-defective. Let fn stand for its dualhypersurface.
• The partial derivatives of fn generate acodimension two perfect ideal of linear type;
• fn is a Koszul free divisor for all n ≥ 3;
Conferencia en Memoria de Pilar Pison Casares – p. 42/50
Sporadic examples, II. Dual of ratio-nal normal curve
The following instance is a lot speculative; it hasbeen verified up to n ≤ 5.
The rational normal curve in Pn is known to be
non-defective. Let fn stand for its dualhypersurface.
• The partial derivatives of fn generate acodimension two perfect ideal of linear type;
• fn is a Koszul free divisor for all n ≥ 3;
• fn is homaloidal if and only if n = 3.
Conferencia en Memoria de Pilar Pison Casares – p. 42/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
The elementary Gordan–Noether polynomialfd ofdegree d+ 1 ≥ 3 is defined as
Conferencia en Memoria de Pilar Pison Casares – p. 43/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
The elementary Gordan–Noether polynomialfd ofdegree d+ 1 ≥ 3 is defined as
(xd0 xd−1
0 x1 . . . xd1) · (xd+2 − xd+1 . . . (−1)d+2x2)
t
Conferencia en Memoria de Pilar Pison Casares – p. 43/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
The elementary Gordan–Noether polynomialfd ofdegree d+ 1 ≥ 3 is defined as
(xd0 xd−1
0 x1 . . . xd1) · (xd+2 − xd+1 . . . (−1)d+2x2)
t
=∑
j=0
(−1)jxd+2−jxd−1−j0 xj
1
Conferencia en Memoria de Pilar Pison Casares – p. 43/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
The elementary Gordan–Noether polynomialfd ofdegree d+ 1 ≥ 3 is defined as
(xd0 xd−1
0 x1 . . . xd1) · (xd+2 − xd+1 . . . (−1)d+2x2)
t
=∑
j=0
(−1)jxd+2−jxd−1−j0 xj
1
• The partial derivatives of fd have a nice recurrencein which ∂fd/∂x0, ∂fd/∂x1 are again elementaryGordan–Noether polynomials, whilst the remainingones are the signed monomials of degree d in x0, x1
Conferencia en Memoria de Pilar Pison Casares – p. 43/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
This makes the Hessian matrix of fd prone to easyinspection; in particular it has rank 4, hence thedimension of the polar image of fd is only 3
Conferencia en Memoria de Pilar Pison Casares – p. 44/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
This makes the Hessian matrix of fd prone to easyinspection; in particular it has rank 4, hence thedimension of the polar image of fd is only 3
Instead, we look for suitable projections to P3 of the
hypersurface V (fd) defined by fd and pose:
Conferencia en Memoria de Pilar Pison Casares – p. 44/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
This makes the Hessian matrix of fd prone to easyinspection; in particular it has rank 4, hence thedimension of the polar image of fd is only 3
Instead, we look for suitable projections to P3 of the
hypersurface V (fd) defined by fd and pose:
Puzzle For a convenient projection Pd+2
99K P3 the
image of V (fd) is a homaloidal surface.
Conferencia en Memoria de Pilar Pison Casares – p. 44/50
Sporadic examples, III. Projecting the elementary
Gordan–Noether hypersurface
This makes the Hessian matrix of fd prone to easyinspection; in particular it has rank 4, hence thedimension of the polar image of fd is only 3
Instead, we look for suitable projections to P3 of the
hypersurface V (fd) defined by fd and pose:
Puzzle For a convenient projection Pd+2
99K P3 the
image of V (fd) is a homaloidal surface.
For low values of d this is directly checkable; ingeneral, it would suffice by a result of Ciliberto et.al. to prove that V (fd) is projectively equivalent tothe dual of the normal scroll of type (1, d).
Conferencia en Memoria de Pilar Pison Casares – p. 44/50
FIFTH PART(apocryphal)
Peripheral questions
Conferencia en Memoria de Pilar Pison Casares – p. 45/50
The depth of the base ideal
Once one starts digging into the more algebraic side of a
Cremona map, one stumbles in the usual questions. We will
restrain ourselves from going to far ashore, but here is a
puzzle:
Conferencia en Memoria de Pilar Pison Casares – p. 46/50
The depth of the base ideal
Once one starts digging into the more algebraic side of a
Cremona map, one stumbles in the usual questions. We will
restrain ourselves from going to far ashore, but here is a
puzzle:
Question Let F : Pn
99K Pn be a Cremona map
defined by forms f ⊂ R = k[x] of degree ≥ 2. When
is the ideal (f) an (x)-saturated ideal?
Conferencia en Memoria de Pilar Pison Casares – p. 46/50
The depth of the base ideal
Once one starts digging into the more algebraic side of a
Cremona map, one stumbles in the usual questions. We will
restrain ourselves from going to far ashore, but here is a
puzzle:
Question Let F : Pn
99K Pn be a Cremona map
defined by forms f ⊂ R = k[x] of degree ≥ 2. When
is the ideal (f) an (x)-saturated ideal?
Remark One reason to ask is that by and large questions on
linear syzygies and fibers go astray to questions on the
cohomological side. It naturally puzzles us to know whether
there are any restrictions of the second nature at all for a
bunch of forms to define a birational map.
Conferencia en Memoria de Pilar Pison Casares – p. 46/50
The depth of the base ideal
Even for n = 2 the above question has a negative answer in
general, as the following result tells us.
Conferencia en Memoria de Pilar Pison Casares – p. 47/50
The depth of the base ideal
Even for n = 2 the above question has a negative answer in
general, as the following result tells us.
Quintics through six points Let pj ∈ P2(1 ≤ j ≤ 6) be
six points in general position in the sense that no three of
them lie on the same line and no conic goes through all six
points. Let Pj ⊂ R = k[x0, x1, x2] denote the prime ideal of
pj and set J :=⋂6
j=1 P2j (“fat points”). Then:
Conferencia en Memoria de Pilar Pison Casares – p. 47/50
The depth of the base ideal
Even for n = 2 the above question has a negative answer in
general, as the following result tells us.
Quintics through six points Let pj ∈ P2(1 ≤ j ≤ 6) be
six points in general position in the sense that no three of
them lie on the same line and no conic goes through all six
points. Let Pj ⊂ R = k[x0, x1, x2] denote the prime ideal of
pj and set J :=⋂6
j=1 P2j (“fat points”). Then:
J is a perfect ideal with free graded resolution
0 −→ R(−7)3 −→ R(−6) ⊕R(−5)3 −→ R −→ R/J ;
Conferencia en Memoria de Pilar Pison Casares – p. 47/50
The depth of the base ideal
Even for n = 2 the above question has a negative answer in
general, as the following result tells us.
Quintics through six points Let pj ∈ P2(1 ≤ j ≤ 6) be
six points in general position in the sense that no three of
them lie on the same line and no conic goes through all six
points. Let Pj ⊂ R = k[x0, x1, x2] denote the prime ideal of
pj and set J :=⋂6
j=1 P2j (“fat points”). Then:
J is a perfect ideal with free graded resolution
0 −→ R(−7)3 −→ R(−6) ⊕R(−5)3 −→ R −→ R/J ;
Any independent 5-forms f0, f1, f2 ∈ J define a plane
Cremona map such that depthR/I = 0, where
I = (f0, f1, f2).Conferencia en Memoria de Pilar Pison Casares – p. 47/50
The depth of the base ideal
No proofs here, but some highlights may be in place:
Conferencia en Memoria de Pilar Pison Casares – p. 48/50
The depth of the base ideal
No proofs here, but some highlights may be in place:
(i) Because of the general position assumption, J is generated
in degrees 5 or higher – this relates to the classical Nagata
lower bound δ(m, r) for the degree of a curve to pass with
multiplicity at least m through r plane points in general
position;
Conferencia en Memoria de Pilar Pison Casares – p. 48/50
The depth of the base ideal
No proofs here, but some highlights may be in place:
(i) Because of the general position assumption, J is generated
in degrees 5 or higher – this relates to the classical Nagata
lower bound δ(m, r) for the degree of a curve to pass with
multiplicity at least m through r plane points in general
position;
(ii) J is generated by 3 quintics f0, f1, f2 and one sextic –
this has been first shown by S. Fitchett(2001);
Conferencia en Memoria de Pilar Pison Casares – p. 48/50
The depth of the base ideal
No proofs here, but some highlights may be in place:
(i) Because of the general position assumption, J is generated
in degrees 5 or higher – this relates to the classical Nagata
lower bound δ(m, r) for the degree of a curve to pass with
multiplicity at least m through r plane points in general
position;
(ii) J is generated by 3 quintics f0, f1, f2 and one sextic –
this has been first shown by S. Fitchett(2001);
(iii) I = J locally off the irrelevant maximal ideal (x), in
particular deg(R/I) = deg(R/J) = 18;
Conferencia en Memoria de Pilar Pison Casares – p. 48/50
The depth of the base ideal
No proofs here, but some highlights may be in place:
(i) Because of the general position assumption, J is generated
in degrees 5 or higher – this relates to the classical Nagata
lower bound δ(m, r) for the degree of a curve to pass with
multiplicity at least m through r plane points in general
position;
(ii) J is generated by 3 quintics f0, f1, f2 and one sextic –
this has been first shown by S. Fitchett(2001);
(iii) I = J locally off the irrelevant maximal ideal (x), in
particular deg(R/I) = deg(R/J) = 18;
(iv) R/I is not Cohen–Macaulay – otherwise a calculation
with Hilbert–Burch matrices in this range yields
deg(R/I) = 19, 21. Conferencia en Memoria de Pilar Pison Casares – p. 48/50
The depth of the base ideal
Some comments to finish:
Conferencia en Memoria de Pilar Pison Casares – p. 49/50
The depth of the base ideal
Some comments to finish:
• Cremona maps in which all the virtual multiplicities
coincide are known as symmetric
Conferencia en Memoria de Pilar Pison Casares – p. 49/50
The depth of the base ideal
Some comments to finish:
• Cremona maps in which all the virtual multiplicities
coincide are known as symmetric
• According to M. Alberich-Carramiñana(LNM, Springer,
2002)there are only four characteristic types of symmetric
Cremona transformations of degree ≥ 2 and exactly one in
degree 5 – however, this knowledge does not seem to yield the
depth value of R/I
Conferencia en Memoria de Pilar Pison Casares – p. 49/50
The depth of the base ideal
Some comments to finish:
• Cremona maps in which all the virtual multiplicities
coincide are known as symmetric
• According to M. Alberich-Carramiñana(LNM, Springer,
2002)there are only four characteristic types of symmetric
Cremona transformations of degree ≥ 2 and exactly one in
degree 5 – however, this knowledge does not seem to yield the
depth value of R/I
• We conjecture that (perhaps, wonder if) there is no such
behaviour in degrees ≤ 4
Conferencia en Memoria de Pilar Pison Casares – p. 49/50
The depth of the base ideal
Some comments to finish:
• Cremona maps in which all the virtual multiplicities
coincide are known as symmetric
• According to M. Alberich-Carramiñana(LNM, Springer,
2002)there are only four characteristic types of symmetric
Cremona transformations of degree ≥ 2 and exactly one in
degree 5 – however, this knowledge does not seem to yield the
depth value of R/I
• We conjecture that (perhaps, wonder if) there is no such
behaviour in degrees ≤ 4
• By Dolgachev there is no such behavior for polar Cremona
maps in degree 2; I know of no “interesting” non-saturated
behavior in higher degrees.Conferencia en Memoria de Pilar Pison Casares – p. 49/50
THE END
THANK YOU!
Conferencia en Memoria de Pilar Pison Casares – p. 50/50