Re: All permutations of a sequence

*To*: mathgroup at smc.vnet.net*Subject*: [mg76323] Re: All permutations of a sequence*From*: "Sem" <sarner2006-sem at yahoo.it>*Date*: Sat, 19 May 2007 04:51:56 -0400 (EDT)*References*: <f2h8t7$v4$1@smc.vnet.net>

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 >