Re: [Maxima-discuss] sign of permutation

[Barton W. <wi...@un...>]

You might like to insert local(arr) as the first line in your code.  Without it, we have

(%i2)  permsign([1,2,3]);
(%o2) 1

(%i3)  arr[2] : 1;
(%o3) 1

(%i4)  permsign([1,2,3]);
(%o4) -1

Also, you might like to either quote nil or change it to false—otherwise, we have

OK
(%i2)  permsign([1,2,3]);
(%o2) 1

Set nil to zero
(%i3)  nil : 0;
(%o3) 0

Not OK
(%i4)  permsign([1,2,3]);
(%o4) 0

--Barton



From: Dimiter Prodanov<mailto:dim...@gm...>
Sent: Saturday, May 6, 2023 4:07 PM
To: Daniel Volinski<mailto:dan...@ya...>
Cc: max...@li...<mailto:max...@li...>
Subject: Re: [Maxima-discuss] sign of permutation

Non-NU Email

Here is what I wrote for the clifford packages

permsign(arr):=block([k:0, len, ret:0 ] ,
     mode_declare([k, len], fixnum),
     if not listp(arr) then return (false),
     len:length(arr),
     for i:1 thru len do (
          if not mapatom(arr[i]) then ret:nil,
          for j:i+1 thru len do
              if ordergreatp(arr[i], arr[j]) then k:k+1
     ),
     if ret#nil then
           if evenp(k)  then 1 else -1
     else 0
);

best regards,

Dimiter

On Sat, May 6, 2023 at 10:00 AM Daniel Volinski via Maxima-discuss <max...@li...<mailto:max...@li...>> wrote:
Hi Jaime,

Thank you for the input.
However, this is not what I need.

My original intention was to symmetrize or anti-symmetrize tensors with abstract indices.
For the symmetrization there is no problem because you sum all the permutations.
But for the anti-symmetrization you need to sum all the permutations but each one
should be multiplied by the sign (or parity) of the permutation.

perm_parity([λ,σ,μ]) issues an error: "argument should be a permutation".

Thanks,

Daniel Volinski


En viernes, 5 de mayo de 2023, 21:23:23 GMT+3, Jaime Villate <vi...@fe...<mailto:vi...@fe...>> escribió:


On 05/05/23 18:24, Daniel Volinski via Maxima-discuss wrote:
> Is there a command in Maxima the calculates the sign of a permutation?
>
> Given a base permutation (a list) and a test permutation (a list) of
> the same elements,
> I would l like a command to calculate the number of switches needed to
> go from one
> to the other or (alternatively) whether this number is odd or even.
>
Hello Daniel,

Function perm_parity, from additional package combinatorics, will give 0
if the permutation is even or 1 if it is odd.

Regards,

Jaime

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