Usage:
<project_from_product I n J
Finds the defining ideal J of the image of a subscheme
of A^n x A^m , minus any component supported on 0 x A^m,
projected to the second factor. IT IS ASSUMED THAT THE
CURRENT RING has at least m variables, and its first m
variables are treated as the variables of A^m. FURTHER,
IT IS ASSUMED THAT THE BASE RING OF I HAS AN ELIMINATION ORDER,
SO THAT ELIM WILL ELIMINATE THE FIRST n VARIABLES.
Parameters:
I = ideal in a ring with n+m variables
n = the number of variables to eliminate
Output values:
J = ideal in the current ring.
The usual application is to compute the projection of a subscheme of
P^(n-1) x A^m or P^(n-1) x P(m-1), given by a bihomogeneous
ideal I, to the second factor.
The method is to saturate the given
ideal with respect to the ideal linear forms
in the first n variables, and then eliminate the
first n variables.