Cremona -- package for some computations on rational maps between projective varieties
Description
Cremona is a package to perform some basic computations on rational and birational maps between absolutely irreducible projective varieties over a field $K$. For instance, it provides general methods to compute degrees and projective degrees of rational maps (see degreeMap and projectiveDegrees) and a general method to compute the push-forward to projective space of Segre classes (see SegreClass). Moreover, all the main methods are available both in version probabilistic and in version deterministic, and one can switch from one to the other with the boolean option Certify.Let $\Phi:X \dashrightarrow Y$ be a rational map from a subvariety $X=V(I)\subseteq\mathbb{P}^n=Proj(K[x_0,\ldots,x_n])$ to a subvariety $Y=V(J)\subseteq\mathbb{P}^m=Proj(K[y_0,\ldots,y_m])$. Then the map $\Phi $ can be represented, although not uniquely, by a homogeneous ring map $\phi:K[y_0,\ldots,y_m]/J \to K[x_0,\ldots,x_n]/I$ of quotients of polynomial rings by homogeneous ideals. These kinds of ring maps, together with the objects of the RationalMap class, are the typical inputs for the methods in this package. The method toMap (resp. rationalMap) constructs such a ring map (resp. rational map) from a list of $m+1$ homogeneous elements of the same degree in $K[x_0,...,x_n]/I$.
Below is an example using the methods provided by this package, dealing with a birational transformation $\Phi:\mathbb{P}^6 \dashrightarrow \mathbb{G}(2,4)\subset\mathbb{P}^9$ of bidegree $(3,3)$.
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We repeat the example using the type RationalMap and using deterministic methods.
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A rudimentary version of Cremona has been already used in an essential way in the paper doi:10.1016/j.jsc.2015.11.004 (it was originally named bir.m2).
Author
- Giovanni Staglianò <[email protected]>
Certification 
Version 4.2.2 of this package was accepted for publication in volume 8 of The Journal of Software for Algebra and Geometry on 11 June 2018, in the article A Macaulay2 package for computations with rational maps. That version can be obtained from the journal or from the Macaulay2 source code repository.
Version
This documentation describes version 5.2.1 of Cremona.
Source code
The source code from which this documentation is derived is in the file Cremona.m2. The auxiliary files accompanying it are in the directory Cremona/.
Exports
-
Types
- RationalMap -- the class of all rational maps between absolutely irreducible projective varieties over a field
-
Functions and commands
- abstractRationalMap -- make an abstract rational map
- approximateInverseMap -- random map related to the inverse of a birational map
- ChernSchwartzMacPherson -- Chern-Schwartz-MacPherson class of a projective scheme
- degreeMap -- degree of a rational map between projective varieties
- EulerCharacteristic -- topological Euler characteristic of a (smooth) projective variety
- exceptionalLocus -- exceptional locus of a birational map
- forceImage -- declare which is the image of a rational map
- forceInverseMap -- declare that two rational maps are one the inverse of the other
- graph -- closure of the graph of a rational map
- inverseMap -- inverse of a birational map
- isBirational -- whether a rational map is birational
- isDominant -- whether a rational map is dominant
- isInverseMap -- checks whether a rational map is the inverse of another
- isMorphism -- whether a rational map is a morphism
- parametrize -- parametrization of a rational projective variety
- point -- pick a random rational point on a projective variety
- projectiveDegrees -- projective degrees of a rational map between projective varieties
- quadroQuadricCremonaTransformation -- quadro-quadric Cremona transformations
- rationalMap -- makes a rational map
- segre -- Segre embedding
- SegreClass -- Segre class of a closed subscheme of a projective variety
- specialCremonaTransformation -- special Cremona transformations whose base locus has dimension at most three
- specialCubicTransformation -- special cubic transformations whose base locus has dimension at most three
- specialQuadraticTransformation -- special quadratic transformations whose base locus has dimension three
- toMap -- rational map defined by a linear system
-
Methods
- abstractRationalMap(PolynomialRing,PolynomialRing,FunctionClosure) -- see abstractRationalMap -- make an abstract rational map
- abstractRationalMap(PolynomialRing,PolynomialRing,FunctionClosure,ZZ) -- see abstractRationalMap -- make an abstract rational map
- abstractRationalMap(RationalMap) -- see abstractRationalMap -- make an abstract rational map
- approximateInverseMap(RationalMap) -- see approximateInverseMap -- random map related to the inverse of a birational map
- approximateInverseMap(RationalMap,ZZ) -- see approximateInverseMap -- random map related to the inverse of a birational map
- approximateInverseMap(RingMap) -- see approximateInverseMap -- random map related to the inverse of a birational map
- approximateInverseMap(RingMap,ZZ) -- see approximateInverseMap -- random map related to the inverse of a birational map
- ChernSchwartzMacPherson(Ideal) -- see ChernSchwartzMacPherson -- Chern-Schwartz-MacPherson class of a projective scheme
- coefficientRing(RationalMap) -- coefficient ring of a rational map
- coefficients(RationalMap) -- coefficient matrix of a rational map
- degree(RationalMap) -- degree of a rational map
- degreeMap(RingMap) -- see degreeMap -- degree of a rational map between projective varieties
- degreeMap(RationalMap) -- degree of a rational map
- degrees(RationalMap) -- projective degrees of a rational map
- multidegree(RationalMap) -- see degrees(RationalMap) -- projective degrees of a rational map
- ? Ideal -- see describe(RationalMap) -- describe a rational map
- describe(RationalMap) -- describe a rational map
- entries(RationalMap) -- the entries of the matrix associated to a rational map
- EulerCharacteristic(Ideal) -- see EulerCharacteristic -- topological Euler characteristic of a (smooth) projective variety
- exceptionalLocus(RationalMap) -- see exceptionalLocus -- exceptional locus of a birational map
- flatten(RationalMap) -- write source and target as nondegenerate varieties
- forceImage(RationalMap,Ideal) -- see forceImage -- declare which is the image of a rational map
- forceInverseMap(RationalMap,RationalMap) -- see forceInverseMap -- declare that two rational maps are one the inverse of the other
- graph(RationalMap) -- see graph -- closure of the graph of a rational map
- graph(RingMap) -- closure of the graph of a rational map
- ideal(RationalMap) -- base locus of a rational map
- image(RationalMap,String) -- closure of the image of a rational map using the F4 algorithm (experimental)
- image(RationalMap) -- see image(RationalMap,ZZ) -- closure of the image of a rational map
- image(RationalMap,ZZ) -- closure of the image of a rational map
- inverse(RationalMap) -- inverse of a birational map
- inverse(RationalMap,Option) -- see inverse(RationalMap) -- inverse of a birational map
- inverseMap(RationalMap) -- see inverseMap -- inverse of a birational map
- inverseMap(RingMap) -- see inverseMap -- inverse of a birational map
- isBirational(RationalMap) -- see isBirational -- whether a rational map is birational
- isBirational(RingMap) -- see isBirational -- whether a rational map is birational
- isDominant(RationalMap) -- see isDominant -- whether a rational map is dominant
- isDominant(RingMap) -- see isDominant -- whether a rational map is dominant
- isInverseMap(RingMap,RingMap) -- see isInverseMap -- checks whether a rational map is the inverse of another
- isInverseMap(RationalMap,RationalMap) -- checks whether two rational maps are one the inverse of the other
- isIsomorphism(RationalMap) -- whether a birational map is an isomorphism
- isMorphism(RationalMap) -- see isMorphism -- whether a rational map is a morphism
- kernel(RingMap,ZZ) -- homogeneous components of the kernel of a homogeneous ring map
- map(RationalMap) -- get the ring map defining a rational map
- map(ZZ,RationalMap) -- see map(RationalMap) -- get the ring map defining a rational map
- matrix(RationalMap) -- the matrix associated to a rational map
- matrix(ZZ,RationalMap) -- see matrix(RationalMap) -- the matrix associated to a rational map
- parametrize(Ideal) -- parametrization of linear varieties and hyperquadrics
- parametrize(PolynomialRing) -- see parametrize(Ideal) -- parametrization of linear varieties and hyperquadrics
- parametrize(QuotientRing) -- see parametrize(Ideal) -- parametrization of linear varieties and hyperquadrics
- point(PolynomialRing) -- see point(QuotientRing) -- pick a random rational point on a projective variety
- point(QuotientRing) -- pick a random rational point on a projective variety
- projectiveDegrees(RingMap) -- see projectiveDegrees -- projective degrees of a rational map between projective varieties
- projectiveDegrees(RationalMap) -- projective degrees of a rational map
- quadroQuadricCremonaTransformation(Ring,ZZ,ZZ) -- see quadroQuadricCremonaTransformation -- quadro-quadric Cremona transformations
- quadroQuadricCremonaTransformation(ZZ,ZZ) -- see quadroQuadricCremonaTransformation -- quadro-quadric Cremona transformations
- quadroQuadricCremonaTransformation(ZZ,ZZ,Ring) -- see quadroQuadricCremonaTransformation -- quadro-quadric Cremona transformations
- rationalMap(List) -- see rationalMap -- makes a rational map
- rationalMap(Matrix) -- see rationalMap -- makes a rational map
- rationalMap(Ring) -- see rationalMap -- makes a rational map
- rationalMap(Ring,Ring) -- see rationalMap -- makes a rational map
- rationalMap(Ring,Ring,List) -- see rationalMap -- makes a rational map
- rationalMap(Ring,Ring,Matrix) -- see rationalMap -- makes a rational map
- rationalMap(RingMap) -- see rationalMap -- makes a rational map
- RationalMap ! -- calculates every possible thing
- compose(RationalMap,RationalMap) -- see RationalMap * RationalMap -- composition of rational maps
- compose(RingMap,RingMap) -- see RationalMap * RationalMap -- composition of rational maps
- RationalMap * RationalMap -- composition of rational maps
- RationalMap ** Ring -- change the coefficient ring of a rational map
- RationalMap == RationalMap -- equality of rational maps
- RationalMap == ZZ -- see RationalMap == RationalMap -- equality of rational maps
- ZZ == RationalMap -- see RationalMap == RationalMap -- equality of rational maps
- RationalMap ^ ZZ -- power
- RationalMap ^* -- see RationalMap ^** Ideal -- inverse image via a rational map
- RationalMap ^** Ideal -- inverse image via a rational map
- RationalMap _* -- direct image via a rational map
- RationalMap Ideal -- see RationalMap _* -- direct image via a rational map
- RationalMap | Ideal -- restriction of a rational map
- RationalMap | Ring -- see RationalMap | Ideal -- restriction of a rational map
- RationalMap | RingElement -- see RationalMap | Ideal -- restriction of a rational map
- RationalMap || Ideal -- restriction of a rational map
- RationalMap || Ring -- see RationalMap || Ideal -- restriction of a rational map
- RationalMap || RingElement -- see RationalMap || Ideal -- restriction of a rational map
- rationalMap(Ideal) -- see rationalMap(Ideal,ZZ,ZZ) -- makes a rational map from an ideal
- rationalMap(Ideal,List) -- see rationalMap(Ideal,ZZ,ZZ) -- makes a rational map from an ideal
- rationalMap(Ideal,ZZ) -- see rationalMap(Ideal,ZZ,ZZ) -- makes a rational map from an ideal
- rationalMap(Ideal,ZZ,ZZ) -- makes a rational map from an ideal
- rationalMap(PolynomialRing,List) -- rational map defined by the linear system of hypersurfaces passing through random points with multiplicity
- rationalMap(Ring,Tally) -- rational map defined by an effective divisor
- rationalMap(Tally) -- see rationalMap(Ring,Tally) -- rational map defined by an effective divisor
- segre(PolynomialRing) -- see segre -- Segre embedding
- segre(QuotientRing) -- see segre -- Segre embedding
- segre(RationalMap) -- see segre -- Segre embedding
- SegreClass(Ideal) -- see SegreClass -- Segre class of a closed subscheme of a projective variety
- SegreClass(RationalMap) -- see SegreClass -- Segre class of a closed subscheme of a projective variety
- SegreClass(RingMap) -- see SegreClass -- Segre class of a closed subscheme of a projective variety
- source(RationalMap) -- coordinate ring of the source for a rational map
- specialCremonaTransformation(Ring,ZZ) -- see specialCremonaTransformation -- special Cremona transformations whose base locus has dimension at most three
- specialCremonaTransformation(ZZ) -- see specialCremonaTransformation -- special Cremona transformations whose base locus has dimension at most three
- specialCremonaTransformation(ZZ,Ring) -- see specialCremonaTransformation -- special Cremona transformations whose base locus has dimension at most three
- specialCubicTransformation(Ring,ZZ) -- see specialCubicTransformation -- special cubic transformations whose base locus has dimension at most three
- specialCubicTransformation(ZZ) -- see specialCubicTransformation -- special cubic transformations whose base locus has dimension at most three
- specialCubicTransformation(ZZ,Ring) -- see specialCubicTransformation -- special cubic transformations whose base locus has dimension at most three
- specialQuadraticTransformation(Ring,ZZ) -- see specialQuadraticTransformation -- special quadratic transformations whose base locus has dimension three
- specialQuadraticTransformation(ZZ) -- see specialQuadraticTransformation -- special quadratic transformations whose base locus has dimension three
- specialQuadraticTransformation(ZZ,Ring) -- see specialQuadraticTransformation -- special quadratic transformations whose base locus has dimension three
- substitute(RationalMap,PolynomialRing,PolynomialRing) -- substitute the ambient projective spaces of source and target
- rationalMap(RationalMap) -- see super(RationalMap) -- get the rational map whose target is a projective space
- super(RationalMap) -- get the rational map whose target is a projective space
- target(RationalMap) -- coordinate ring of the target for a rational map
- toExternalString(RationalMap) -- convert to a readable string
- toMap(Ideal) -- see toMap -- rational map defined by a linear system
- toMap(Ideal,List) -- see toMap -- rational map defined by a linear system
- toMap(Ideal,ZZ) -- see toMap -- rational map defined by a linear system
- toMap(Ideal,ZZ,ZZ) -- see toMap -- rational map defined by a linear system
- toMap(List) -- see toMap -- rational map defined by a linear system
- toMap(Matrix) -- see toMap -- rational map defined by a linear system
- toMap(RingMap) -- see toMap -- rational map defined by a linear system
-
Symbols
- BlowUpStrategy
- Certify -- whether to ensure correctness of output
- CodimBsInv
- Dominant
- NumDegrees