morphism between permutation group and matrix group

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Sage did not remove Gap's functionality, it simply did not interface to it. There seems to be no implementation of Homsets specific to groups, and in particular no implementation of morphisms specified via the images of the generators, like there is for rings, for example. I agree that the error message is not very informative.

I see no easy workaround: either you define your own python functions, or you work directly with gap. This is probably worth a ticket on trac.sagemath.org.

Hi,

It is currently not possible to set group morphisms from generators (see also question 3157). Nevertheless, you can build a morphism by explicitely give the corresponding map

sage: flip = PermutationGroupElement("(1,2)")
sage: g = PermutationGroup([flip])
sage: id3  = Matrix(GF(3), 1, 1, [1])
sage: flop = Matrix(GF(3), 1, 1, [2])
sage: k = MatrixGroup([flop])
sage: H = Hom(g,k)
sage: h = H(lambda x: flop if x == flip else id3)

And then

sage: h
Generic morphism:
  From: Permutation Group with generators [(1,2)]
  To:   Matrix group over Finite Field of size 3 with 1 generators ([2],)
sage: h(flip) == flop
True

In 3157 you can see how to do this for arbitrary finite groups using the Cayley graph.