Re: [sage-devel] Re: Bug in category_singleton.__contains__ or in CombinatorialFreeModule

['Reimundo Heluani' via sage-devel]

On Jun 20, Markus Wageringel wrote:

I think the problem here is that enumerating over a disjoint union of an
infinite set and a finite set will never reach the elements in the finite set.
There is this comment in the documentation of DisjointUnionEnumeratedSets


      Possible extensions: the current enumeration order is not suitable
      for unions of infinite enumerated sets (except possibly for the
      last one). One could add options to specify alternative enumeration
      orders (anti-diagonal, round robin, ...) to handle this case.


so the workaround is to first enumerate over the finite set and then the
infinite set, as the last enumerated set in the union will determine whether
the disjoint union is finite or not:


True

False

sage: DisjointUnionEnumeratedSets([NonNegativeIntegers(), Family('x')]) in FiniteSets() sage: DisjointUnionEnumeratedSets([Family('x'), NonNegativeIntegers()]) in FiniteSets()

Nice! I hadn't seen that. I do think however that the check in CombinatorialFreeModule should be changed or at least that taking the disjoint union should print a warning that that this set is considered to be finite. Best, R.

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