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The embedding of a magma into the magma with zero adjoined was built with the four-argument `MappingByFunction`, which marks the map bijective. The zero is not in its image, so `IsBijective` returned true and `PreImage( map, zero )` returned `fail` instead of raising an error. Build it with the `false, prefun` form and mark it injective and not surjective. The internal `_InjectionPrincipalFactor` had the same flaw for Rees 0-matrix semigroups; `IsomorphismReesZeroMatrixSemigroup` now uses `PreImagesRepresentativeNC` instead of the inverse mapping. Assisted-by: Generative AI
fingolfin
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- Coverage 79.67% 79.67% -0.01%
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Lines 295465 295470 +5
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fingolfin
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Oct 18, 2026
`UnderlyingInjectionZeroMagma` is injective but not surjective: the adjoined zero is not in its image. GAP currently marks it bijective by mistake, which makes `x ^ InverseGeneralMapping( inj )` work. Once GAP fixes that (gap-system/gap#6767), the inverse is no longer total and `^` raises an error, breaking `IsomorphismPartialPermSemigroup` for zero groups. Use `PreImagesRepresentative` instead, which works with both old and fixed GAP versions. Assisted-by: Generative AI
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The embedding of a magma into the magma with zero adjoined was built with the four-argument
MappingByFunction, which marks the map bijective. The zero is not in its image, soIsBijectivereturned true andPreImage( map, zero )returnedfailinstead of raising an error. Build it with thefalse, prefunform and mark it injective and not surjective.The internal
_InjectionPrincipalFactorhad the same flaw for Rees 0-matrix semigroups;IsomorphismReesZeroMatrixSemigroupnow usesPreImagesRepresentativeNCinstead of the inverse mapping.Assisted-by: Generative AI
This leads to a test failure in the Semigroups package (as it relied on this bug, accidentally); fix for that is
being prepared nowat semigroups/Semigroups#1238.