#20555: descents for Permutations : cleanup
-------------------------+-------------------------------------------------
Reporter: | Owner:
chapoton | Status: needs_review
Type: | Milestone: sage-7.2
enhancement | Resolution:
Priority: major | Merged in:
Component: | Reviewers: Travis Scrimshaw, Christian Stump
combinatorics | Work issues:
Keywords: | Commit:
permutation | 8dc44205ed34a559df49aa24ddd14facf47a7b0e
Authors: | Stopgaps:
Frédéric Chapoton |
Report Upstream: N/A |
Branch: |
u/chapoton/20555 |
Dependencies: |
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Comment (by stumpc5):
Replying to [comment:26 tscrim]:
> As a counterpoint, the (very) good reasons that descents start at 0 is
because Python uses 0-based indexing:
> {{{
> sage: p = Permutation([5,3,2,4,1])
> sage: p.descents()
> [0, 1, 3]
> sage: p[0]
> 5
> }}}
> This would break this behavior with the current branch, and actually,
thinking a bit more now about it, we should add some documentation to this
point.
> (I am pretty sure the position of the descent must correspond to the
position of the larger value for the related math to work, but it's been
awhile since I've thought about these things.)
I do totally agree with the math part. But I am less convinced of the sage
part:
{{{
sage: p = Permutation([5,3,2,4,1])
sage: p[0]
5
}}}
I would consider {{{p[0]}}} an implementation detail (as this might mean
something in one-line notation, or the first cycle in cycle decomposition,
or whatever else, depending on the implementation of permutations) and not
a mathematically correct operation. In my eyes, the mathematically correct
operation is
{{{
sage: p(1)
5
}}}
which matches Frédéric's indexing.
--
Ticket URL: <http://trac.sagemath.org/ticket/20555#comment:28>
Sage <http://www.sagemath.org>
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