Fix isomorphisms from fp monoid to fp semigroup #1039
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Previously the inverse of the returned isomorphism was not properly defined (and not the inverse of the isomorphism), I rewrote the function while fixing it. Also added a test which highlighted the previous problem.
A semigroup can be isomorphic to a monoid, while not being a monoid in the technical GAP sense. For example, the semigroup generated by Transformation([1, 2, 3, 3, 3]) is not a monoid in the GAP sense but is mathematically. With this change we could now install methods for IsomorphismFpMonoid for such a semigroup.
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The first commit in this PR is whitespace changes only. I should not have done this since the functional changes are hidden in all of the rewriting, but I did not find time to redo this since I first found this bug and suggested this fix in June. |
Current coverage is 49.60% (diff: 97.50%)@@ master #1039 diff @@
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Files 424 424
Lines 223277 223278 +1
Methods 3430 3430
Messages 0 0
Branches 0 0
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+ Hits 110656 110765 +109
+ Misses 112621 112513 -108
Partials 0 0
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james-d-mitchell
changed the title
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Thanks! And you kept the whitespace changes in a separate commit, so it was possible to review the other two commits without being distracted by them. I think that's fine. |
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There are two main changes in this PR:
Fix a bug in the method for
IsomorphismFpSemigroupfor a finitely presented monoid. Previously the inverse of the returned isomorphism was not properly defined (and not the inverse of the isomorphism)Change the declaration of
IsomorphismFpMonoidso that it can be applied to objects inIsSemigrouprather than onlyIsMonoid. A semigroup can be isomorphic to a monoid, while not being a monoid in the technical GAP sense. For example, the semigroup generated byTransformation([1, 2, 3, 3, 3]) is not a monoid in the GAP sense but is mathematically. With this change we could now install methods for
IsomorphismFpMonoidfor such a semigroup.