Re: [Maxima-discuss] sign of permutation

[Andrii S. <and...@gm...>]

> You might like to insert local(arr) as the first line in your code.

A bit off-topic, but:

I always wondered, why Maxima does not save the properties of an argument, just like it saves its value? See also the bug #3251 at https://sourceforge.net/p/maxima/bugs/3251/.


Andrii

> 7 трав. 2023 р. о 14:35 Barton Willis via Maxima-discuss <max...@li...> написав(ла):
> 
> 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
> <E73FF49F87D646DAA6E7B7B891733959.png>
> 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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