On Fri, Sep 05, 2014 at 07:58:30AM -0700, Dima Pasechnik wrote:
>    I just had a quick look at it, and the following looks downright scary:

>    +            For backward compatibility, if a permutation group element
>    +            acts on the integers `\{1,\ldots,n\}` or a subset thereof,
>    +            the columns are considered as being numbered from `1`::
>    +
>    +                sage: G = PermutationGroup(['(1,2,3)(4,5)',
>    '(1,2,3,4,5)'])
>    +                sage: sigma, tau = G.gens()
>    +                sage: sigma
>    +                (1,2,3)(4,5)
>    +                sage: M = copy(N)
>    +                sage: M.permute_columns(sigma)
>    +                sage: M
>    +                [1 0 1 0 0]
>    +                [2 2 0 0 0]
>    +                [0 3 0 0 3]
>    +                [0 0 0 4 4]
>    +                [0 0 0 5 0]
>    note that there is an unpleasant ambiguity here:
>    namely, if, say in this example you take
>    G = PermutationGroup(['(1,2,3)'])
>    how one could tell whether it is meant to permute stuff in [0..2], or
>    in [1..3] ?
>    Do you mean that one must write G = PermutationGroup(['(0),(1,2,3)'])
>    for the latter?

I agree it's not great. But do you have a better proposal?

The current situation is a painful source of confusion, and is
actually already ambiguous if one is not reading the specifications
carefully: in your above example, the unwary user may well expect that
he will permute stuff in [1..3].

The ticket only adds a new feature allowing for the natural 0-based
permutations. Otherwise it does not change the current behavior.

Cheers,
                                Nicolas
--
Nicolas M. ThiƩry "Isil" <nthi...@users.sf.net>
http://Nicolas.Thiery.name/

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