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Fix Embedding and Projection of pc group direct products to not error or return corrupted results - #6790
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fingolfin wants to merge 1 commit into
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Fix Embedding and Projection of pc group direct products to not error or return corrupted results#6790fingolfin wants to merge 1 commit into
Embedding and Projection of pc group direct products to not error or return corrupted results#6790fingolfin wants to merge 1 commit into
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For a direct product D of pc groups, `Embedding` and `Projection` took the generators of each factor from `Pcgs(D)`, at their positions in the defining presentation. If `Pcgs(D)` is not the family pcgs, they mapped to the wrong elements, and the embedding was not even a homomorphism: after `SetPcgs(dp, SpecialPcgs(FamilyPcgs(dp)))` for the direct product of the extraspecial group 2^3+ and C4, `Embedding(dp, 2)` mapped C4 onto a Klein four group. Before #6002, `SpecialPcgs` replaced `Pcgs(dp)` in this way, which is how #6000 arose. Use `FamilyPcgs(D)`, whose order is that of the presentation. Pc group semidirect products are not affected, as their `Pcgs` is set to the family pcgs on construction. Fixes #6000 Assisted-by: Generative AI
fingolfin
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kind: bug
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #6790 +/- ##
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- Coverage 79.72% 79.72% -0.01%
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Files 684 684
Lines 295592 295587 -5
Branches 8660 8660
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- Hits 235650 235642 -8
Misses 58126 58126
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For a direct product D of pc groups,
EmbeddingandProjectiontook the generators of each factor fromPcgs(D), at their positions in the defining presentation. IfPcgs(D)is not the family pcgs, they mapped to the wrong elements, and the embedding was not even a homomorphism: afterSetPcgs(dp, SpecialPcgs(FamilyPcgs(dp)))for the direct product of the extraspecial group 2^3+ and C4,Embedding(dp, 2)mapped C4 onto a Klein four group. Before #6002,SpecialPcgsreplacedPcgs(dp)in this way, which is how #6000 arose.Use
FamilyPcgs(D), whose order is that of the presentation. Pc group semidirect products are not affected, as theirPcgsis set to the family pcgs on construction.Fixes #6000
Assisted-by: Generative AI