Two fixes for fp group homomorphisms and Improvement for Fitting free matrix group setup #3103
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You write 2x
"
gi:=MappingGeneratorsImages(hom)[2]{ListPerm(PermList(hom!.genpositions)^-1,
Length(hom!.genpositions))};". .
I think it would be much more efficient as
gi{hom!.genpositions} := MappingGeneratorsImages (hom)[2];
I.e. without creating extra permutations and lists.
…On Wed, Dec 12, 2018, 09:33 Alexander Hulpke ***@***.*** wrote:
@hulpke <https://github.com/hulpke> requested your review on: #3103
<#3103> Two fixes for fp group
homomorphisms and Improvement for Fitting free matrix group setup.
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fingolfin
reviewed
Dec 12, 2018
| @@ -28,7 +28,17 @@ local cache,ffs,pcisom,rest,it,kpc,k,x,ker,r; | |||
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| pcisom:=ffs.pcisom; | |||
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| rest:=RestrictedMapping(ffs.factorhom,U); | |||
| #rest:=RestrictedMapping(ffs.factorhom,U); | |||
| if IsPermGroup(U) and AssertionLevel()>1 then | |||
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The commit message says:
using
RestrictedMappingas it will not work for certain matrix group homomorphisms
So, does that mean that there is a bug in RestrictedMapping? Or is this more a general problem where e.g. RestrictedMapping doesn't have enough information to do something sensible in all cases? Or perhaps a third option?
Also, what exactly is a "matrix group homomorphisms" in this context? A hom with a matrix group as source? Range? Both? Either? And, based on that, I'd have expected that only these kinds of homs would be treated separately here, and the rest handled by RestrictedMapping anyway, but that's not the case. I am sure there is a good reason, but what is it?
In any case, IMHO there should be a comment here that briefly explains these things.
lib/fitfree.gi
Outdated
| List(GeneratorsOfGroup(U),x->ImagesRepresentative(ffs.factorhom,x))); | ||
| RUN_IN_GGMBI:=false; | ||
| fi; | ||
| if rest=fail then Error("can't build homomorphism"); fi; |
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So when would this ever fail? Can we perhaps give a slightly more helpful error message?
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I've replaced the error with an assertion.
Codecov Report
@@ Coverage Diff @@
## master #3103 +/- ##
==========================================
+ Coverage 83.56% 83.57% +0.01%
==========================================
Files 686 686
Lines 336759 336772 +13
==========================================
+ Hits 281396 281466 +70
+ Misses 55363 55306 -57
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Codecov Report
@@ Coverage Diff @@
## master #3103 +/- ##
==========================================
+ Coverage 83.55% 83.57% +0.01%
==========================================
Files 686 686
Lines 336665 336777 +112
==========================================
+ Hits 281317 281460 +143
+ Misses 55348 55317 -31
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@laurentbartholdi |
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laurentbartholdi
approved these changes
Dec 13, 2018
for the epimorphism from a group to its fitting free factor, as no method for constructing this restriction might exist. (E.g. in the case of matrix groups.)
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This fixes #3097, #3100 and a nuisance error when working with matrix groups under the fitting free setup (avoid
RestrictedMappingwhich runs into trouble).