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MultiprojectiveVarieties :: inverse2

inverse2 -- inverse of a birational map using a faster algorithm for a special class of maps

Synopsis

Description

This assumes that the graph of the input map Phi is defined by a simplified system of equations, which may not be true. If the option Verify is set to true, which is the default choice, then it is verified that the left and right composition of Phi with the returned map is the identity, throwing an error if this is not the case.

i1 : K = ZZ/10000019;
i2 : -- map defined by the cubics through the secant variety to the rational normal curve of degree 6
     Phi = multirationalMap rationalMap(ring PP_K^6,Grass(2,4,K),gens ideal PP_K([6],2));

o2 : MultirationalMap (rational map from PP^6 to GG(2,4))
i3 : time Psi = inverse2 Phi;
     -- used 0.394577 seconds

o3 : MultirationalMap (birational map from GG(2,4) to PP^6)
i4 : assert(Phi * Psi == 1)
i5 : Phi' = Phi || Phi;

o5 : MultirationalMap (rational map from PP^6 x PP^6 to GG(2,4) x GG(2,4))
i6 : time Psi' = inverse2 Phi';
     -- used 1.45081 seconds

o6 : MultirationalMap (birational map from GG(2,4) x GG(2,4) to PP^6 x PP^6)
i7 : assert(Phi' * Psi' == 1)

See also

Ways to use inverse2 :

For the programmer

The object inverse2 is a method function.