an ideal, the ideal defining the closure of the image of Phi, or its degree d homogeneous component if d is passed
Description
This computation is done through the kernel of a ring map representing the rational map. See kernel(RingMap) (missing documentation) and kernel(RingMap,ZZ) for more details.
See also
kernel(RingMap,ZZ) -- homogeneous components of the kernel of a homogeneous ring map
kernel(RingMap) (missing documentation) -- kernel of a ringmap