#19737: SetPartition.to_permutation().cycle_tuples() is not the identity
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Reporter: mantepse | Owner:
Type: defect | Status: needs_review
Priority: major | Milestone: sage-7.0
Component: combinatorics | Resolution:
Keywords: set partitions, | Merged in:
permutations | Reviewers:
Authors: Martin Rubey | Work issues:
Report Upstream: N/A | Commit:
Branch: | c6a3939d2e18efe3906e1f959dc5c019c921a944
u/mantepse/setpartition_to_permutation___cycle_tuples___is_not_the_identity|
Stopgaps:
Dependencies: |
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Comment (by mantepse):
Replying to [comment:15 ncohen]:
> > Oh, I missed that, I thought that Permutation would handle bijections,
not just Permutations of `{1,...,n}`. That's really strange, I must say.
> >
> > Hm, now I'm not sure what should be done. I guess, the only
reasonable thing then is to fail when the ground set is not of this form.
Well, that might make sense anyway.
>
> On the sage-devel thread I opened (se [comment:6]) I proposed to rename
the class (or to create a new one) which checks from the start that its
ground set is 1...n. This way, one could write methods like
`to_permutation` without having to check it manually, each time.
>
> There is a usage from a `SetPartition` class with arbitrary ground set
(I do have examples in mind) and usage for a specific
`IntegerSetPartition` class restricted to those 1...n integers, which is
the only case many seem to be interested in.
>
> Mixing the two, as it is done in this class, is a bad idea.
I am not quite convinced that this is true. But if you are convinced, is
there an easy way to do it? Can I simply do
{{{
class IntegerSetPartition(SetPartition):
def to_permutation(self):
....
}}}
where should I put the check that the ground set is {1,...,n}?
--
Ticket URL: <http://trac.sagemath.org/ticket/19737#comment:18>
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