Re: All permutations of a sequence

• To: mathgroup at smc.vnet.net
• Subject: [mg76333] Re: All permutations of a sequence
• From: "Sem" <sarner2006-sem at yahoo.it>
• Date: Sun, 20 May 2007 02:20:04 -0400 (EDT)
• References: <f2h8t7\$v4\$1@smc.vnet.net> <f2mful\$mm7\$1@smc.vnet.net>

```Sorry, the loading of the package 'Combinatorica' isn't necessary for the
function.

"Sem"
> Hi Virgil,
>
> the above code produces all the permutations of a list, except those that
> reverse their ordering.
> The function is recursive but I suppose that an iterative algorithm can be
> wrote easily, i.e. for large list.
>
> << DiscreteMath`Combinatorica`
>
> NotReversingPermutations[l_List] := {l} /; Length[l] <= 2
>
> NotReversingPermutations[l_List] :=
>    Module[
>                {e = Last[l], n = Length[l], r},
>                r = Table[Insert[#, e, -i], {i, 1, n}] & /@
> NotReversingPermutations[Drop[l, -1]];
>                Flatten[r, 1]
>                ];
>
> HTH
> Regards,
>
>    Sem
>
>>"Virgil Stokes"
>>I would like to get all unique permutations of a sequence (e.g.
>> {1,2,3,4,5,6,7,8,9,10,11,12,13}) with the constraint that all sequences
>> that have an opposite ordering are not included (e.g. {13, 12,
>> 11,10,9,8,6,5,4,3,2,1}) would not be included.
>>
>> Are there some Mathematica commands that can be used to efficiently
>> generate these permutations?
>>
>> --V. Stokes
>>
>
>

```

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