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RationalMaps :: RationalMaps

RationalMaps -- rational maps


RationalMaps is a package for computing things related to maps between projective varieties.

It focuses on finding where a birational map is undefined, checking whether a map is a closed embedding, checking birationality and computing inverse maps

Mathematical background:
Functionality overlap with other packages:

Parametrization.m2: While the packageParametrization focuses on mostly on curves, it also includes a function invertBirationalMap which has the same functionality as inverseOfMap. On the other hand, these two functions were implemented somewhat differently and so sometimes one function can be substantially faster than the other.

Cremona.m2: The package Cremona focuses on fast probabilistic computations in general cases and deterministic computations for special kinds of maps from projective space. More precisely,



This documentation describes version 0.3 of RationalMaps.

Source code

The source code from which this documentation is derived is in the file RationalMaps.m2.


  • Functions and commands
    • baseLocusOfMap -- Computes base locus of a map from a projective variety to projective space
    • idealOfImageOfMap -- Finds defining equations for the image of a rational map between varieties or schemes
    • inverseOfMap -- Computes the inverse map of a given birational map between projective varieties. Returns an error if the map is not birational onto its image.
    • isBirationalMap -- Checks if a map between projective varieties is birational.
    • isBirationalOntoImage -- Checks if a map between projective varieties is birational onto its image.
    • isEmbedding -- Checks whether a map of projective varieties is a closed embedding.
    • isRegularMap -- Checks whether a map to projective space is regular
    • isSameMap -- Checks whether two maps to projective space are really the same
    • jacobianDualMatrix -- Computes the Jacobian Dual Matrix, a matrix whose kernel describing the syzygies of the inverse map.
    • mapOntoImage -- Given a map of rings, correspoing to X mapping to Y, this returns the map of rings corresponding to X mapping to f(X).
    • sourceInversionFactor -- Computes the the common factor among the the components of the composition of the inverse map and the original map.
  • Symbols
    • AssumeDominant -- If true, certain functions assume that the map from X to Y is dominant.
    • CheckBirational -- If true, functions will check birationality.
    • HybridLimit -- An option to control HybridStrategy
    • HybridStrategy -- A strategy for inverseOfMap, isBirationalMap and isEmbedding.
    • MinorsCount -- An option controlling the behavior of isBirational and inverseOfMap (and other functions which call those).
    • QuickRank -- An option for computing how rank is computed
    • ReesStrategy -- A strategy for inverseOfMap, isBirationalMap, and is Embedding.
    • SaturateOutput -- If false, certain functions will not saturate their output.
    • SaturationStrategy -- A strategy for inverseOfMap, isBirationalMap, isEmbedding.
    • SimisStrategy -- A strategy for inverseOfMap, isBirationalMap and isEmbedding.