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Something with automorphism groups hanging #3167

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thofma opened this issue Jan 10, 2024 · 5 comments
Closed

Something with automorphism groups hanging #3167

thofma opened this issue Jan 10, 2024 · 5 comments
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bug Something isn't working topic: groups

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@thofma
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thofma commented Jan 10, 2024

The following hangs for me:

julia> F = free_group(["x", "y"])
Free group of rank 2

julia> (x, y) = gens(F);

julia> Q, = quo(F, [x^-3*y^-3*x^-1*y*x^-2,  x*y^-1*x^-1*y*x^-1*y^-1*x^3*y^-1, x*y^-1*x^-1*y^2*x^-2*y^-1*x^2]);

julia> A = automorphism_group(Q);

julia> H, HtoA = inner_automorphism_group(A);

julia> Q, AtoQ = quo(A, H);

julia> [preimage(AtoQ, q) for q in Q] #this will hang forver

Any idea @ThomasBreuer? I tried adding check = false, but it does not exist for the argument types.

@thofma thofma added the bug Something isn't working label Jan 10, 2024
@ThomasBreuer
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Q has order 16.
I can reproduce that the primage computation for some of the elements takes very long.
This happens inside the GAP function PreImagesRepresentative. I will try to find a fix for that.

@ThomasBreuer
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@thofma Concerning check = false, one can enter [haspreimage(AtoQ, q, check = false) for q in Q].
Currently the documentation for preimage suggests this if one wants to omit the membership tests.
If you thinkk that it is better to support check = false also for preimage then we can easily change this.

#3190 yields already some improvement, but there is still the problem from gap-system/gap/issues/5590.

@thofma
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thofma commented Jan 16, 2024

Thanks for the explanation. I don't mind too much about the check = false. I was just wondering about the inconsistency in the keyword arguments of preimage/haspreimage.

@thofma
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thofma commented Jan 16, 2024

Should we close this and keep the discussion/progress at #3171?

@ThomasBreuer
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@thofma

Should we close this and keep the discussion/progress at #3171?

Yes, #3171 is more general.

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Labels
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