sorry some keyboard shortcut has sent the post. I was saying:
def C(*args): #C for "cycle"
return Permutation([ args ])
and then for (1, 2, 3)(4, 5) I type C(1, 2, 3) * C(4, 5). I got very used
to it.
cheers
Pierre
On Thursday, September 11, 2014 2:12:21 PM UTC+2, Pierre wrote:
>
> Hi all,
>
> Here are a few suggestions:
>
> (1) permutations have a to_matrix() method; permuting the rows of M
> according to the permutation sigma can be done by
>
> M= M * sigma.to_matrix()
>
> and for the rows:
>
> M= sigma.to_matrix().transpose() * M
>
> Of course there is a debate about the behaviour of to_matrix() ; however i
> think the code for matrices should rely on to_matrix(), and then at least
> the conversation would be strictly one about the code for permutations.
> This answers a complain above.
>
> (2) if sigma is a permutation, i don't think sigma[i] should make sense at
> all. This requires the user to appreciate that sigma is implemented as a
> list, which i think is not a good idea. Of course some people (including
> me) want the permutation as a word; why not have a as_word() method, which
> could take a bunch of parameters for shifting (starting from 0 instead of 1
> for example...) ?
>
> Also, permutations should always print in cycle notation, just like in GAP
> ! It's much easier to read, and drives the user away from the idea that the
> permutation "is" a list.
>
> [By the way, i also don't think sigma(i) should be implemented. The
> permutations in Sage act on the right, so i^sigma, just like in GAP, is
> more logical. This is not a very useful comment as changing the behaviour
> of standard ints is complicated (if possible at all?), and backward
> compatibility is needed.]
>
> (3) in fact i agree that permutations should not be implemented as lists,
> and should be thought of as acting on all integers from 0, as was
> suggested. Would the multiplication be as fast if they were stored as lists
> of cycles? I'm thinking that an implementation as a dictionnary, much like
> a sparse matrix, would be reasonable.
>
> (4) Here's a notation I use when I have a lot of permutation to input
> interactively:
>
> def C(*args) : # C for "cycle"
>
>
>
>
>
>
>
>
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