precomputedHyperellipticPoint -- compute a point in the hyperelliptic locus using the unirational parametrization
Synopsis
Usage:
phi = precomputedHyperellipticPoint(Sa)
Inputs:
Sa, a ring, the polynomial ring containing the a-variables
Outputs:
phi, a ring map, from the ring Sa to the Cox ring of the parametrization space
Description
The hyperelliptic locus Vhyp in Q ⊂ℙ11 is is birational to a product of a Hirzebruch surface F with 3 copies of ℙ1. The procedure returns a ring map corresponding to the rational map