Change IsomorphismPermGroupForMatrixGroup to always use NiceMonomorphism if available#6244
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fingolfin merged 1 commit intogap-system:masterfrom Mar 23, 2026
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Use a known `NiceMonomorphism` also in the case that its image is *not* a permutation group. The idea is: We trust GAP that the image is nicer than the given group, in the sense that computing an `IsomorphismPermGroup` for the image is easier. An example is that the given group is a matrix group in characteristic zero and its `NiceMonomorphism` is a reduction to a matrix group over a finite field. (This strategy works only because `NiceMonomorphism` guarantees an image that is really nicer. Note that other functions do not have this property, for example `SmallerDegreePermutationRepresentation` is allowed to return an identity mapping.)
ThomasBreuer
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fingolfin
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IsomorphismPermGroupForMatrixGroupIsomorphismPermGroupForMatrixGroup to always use NiceMonomorphism if available
fingolfin
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Somehow I missed this PR ... (we have too many in flight PRs above this one right now... gotta cull this list)
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Use a known `NiceMonomorphism` also in the case that its image is *not* a permutation group. The idea is: We trust GAP that the image is nicer than the given group, in the sense that computing an `IsomorphismPermGroup` for the image is easier. An example is that the given group is a matrix group in characteristic zero and its `NiceMonomorphism` is a reduction to a matrix group over a finite field. (This strategy works only because `NiceMonomorphism` guarantees an image that is really nicer. Note that other functions do not have this property, for example `SmallerDegreePermutationRepresentation` is allowed to return an identity mapping.)
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Use a known
NiceMonomorphismalso in the case that its image is not a permutation group.The idea is:
We trust GAP that the image is nicer than the given group, in the sense that computing an
IsomorphismPermGroupfor the image is easier.An example is that the given group is a matrix group in characteristic zero and its
NiceMonomorphismis a reduction to a matrix group over a finite field.(This strategy works only because
NiceMonomorphismguarantees an image that is really nicer.Note that other functions do not have this property, for example
SmallerDegreePermutationRepresentationis allowed to return an identity mapping.)(The same change has been implemented in oscar-system/Oscar.jl/pull/5791.)