h = mapOntoImage(f)
psi = mapOntoImage(phi)
Given $f : X \to Y$ mapOntoImage returns $X \to \overline{\phi(X)}$. Alternately, given $f: S \to R$, mapOntoImage just returns $S/(kernel f) \to R$. mapOntoImage first computes whether the kernel is $0$ without calling ker, which can have speed advantages.
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The object mapOntoImage is a method function with options.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/RationalMaps.m2:1935:0.