Function: ffinvmap
Section: number_theoretical
C-Name: ffinvmap
Prototype: G
Help: ffinvmap(m): given a map m between finite fields, return a partial map
that return the pre-images by the map m.
Doc: $m$ being a map from $K$ to $L$ two finite fields, return the partial map
$p$ from $L$ to $K$ such that for all $k\in K$, $p(m(k))=k$.
\bprog
? a = ffgen([3,5],'a);
? b = ffgen([3,10],'b);
? m = ffembed(a, b);
? p = ffinvmap(m);
? u = random(a);
? v = ffmap(m, u);
? ffmap(p, v^2+v+2) == u^2+u+2
%7 = 1
? ffmap(p, b)
%8 = []
@eprog