Re: efficient programming problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg84154] Re: efficient programming problem*From*: DrMajorBob <drmajorbob at bigfoot.com>*Date*: Wed, 12 Dec 2007 01:14:29 -0500 (EST)*References*: <3584443.1197383067825.JavaMail.root@m35>*Reply-to*: drmajorbob at bigfoot.com

Maybe you should use your own "product" function; you can always change it to Times when everything else is done. (And you don't need Unevaluated.) Your application may need more rules or a more general version of the second rule I added for "prod", but maybe this will get you down the road a bit: Clear[wick, pair, prod] prod[___, 0, ___] = 0; prod[pair[z1_[w_], z2_[x_]], pair[z3_[y_], z4_[z_]]] /; z == x + 1 && z2 == z4 := prod[pair[z1[w], z2[z]], pair[z3[y], z4[x]]] wick[a_, b_] := pair[a, b]; wick[a_, b__] := Sum[prod[pair[a, {b}[[i]]], wick @@ Delete[{b}, i]], {i, Length@{b}}]; pair[c[_], c[_]] := 0 pair[d[_], d[_]] := 0 wick[c[1], c[2], d[3], d[4]] /. prod -> Times 2 pair[c[1], d[4]] pair[c[2], d[3]] Bobby On Tue, 11 Dec 2007 05:12:35 -0600, Michael Weyrauch <michael.weyrauch at gmx.de> wrote: > Hello, > > I am generating a sum of a product of "pairs" using the following rules > (partly stolen from an earlier post of Carl Woll in this group.) > > wick[a_, b_] := pair[a, b]; > > wick[a_, b__] := Sum[Expand[pair[a, = > {b}[[i]]]*Delete[Unevaluated[wick[b]], i]], {i, Length[{b}]}]; > > pair[c[_], c[_]] := 0 > > pair[d[_], d[_]] := 0 > > So, when I evaluate > > wick[c[1], c[2], d[3], d[4]] > > I get > > pair[c[1], d[4]]*pair[c[2], d[3]] + pair[c[1], d[3]]*pair[c[2], d[4]] > > which is the desired sum of product of pairs. > > Now, I would like to implement an additional rule which holds for my > pairs, namely that it is allowed to interchange the parameters > (1,2) or (3,4), so that the above sum of pairs simplifies e.g. to > > 2*pair[c[1], d[4]]*pair[c[2], d[3]] > > A more complicated example would be to evaluate > > wick[c[1], c[2], d[3], d[4], c[5], c[6], d[7], d[8]] > > which produces a sum of 24 products of pairs. Now if I again implement > the additional rule that one is allowed to interchange (1,2) > or (3,4) or (5,6) ( and so on) one obtains only a sum of 3 products of = > pairs which reads > > 4*pair[c[1], d[3]]*pair[c[2], d[4]]*pair[c[5], d[7]]*pair[c[6], d[8]] + > > 16*pair[c[1], d[7]]*pair[c[2], d[3]]*pair[c[5], d[8]]*pair[d[4], c[6]] + > > 4*pair[c[1], d[7]]*pair[c[2], d[8]]*pair[d[3], c[5]]*pair[d[4], c[6]] > > I am looking for a method which implements this (1,2) (3,4) (5,6)... > interchange symmetry in a memory and time efficient way, so > that Mathematica directly produces the correctly "reduced" sum of > products of pairs as exemplified above for simple cases. > > I have now tried two implementations, but both use too much memory and > time (this is relevant of course only for much longer > examples which contain say 4 times as many c[] and d[] in wick[] than > the examples given), since my methods essentially first > generate the some of all products of pairs generated by the rules given > in the beginning of this post and then analyze the sum of > terms and reduce them according to the additional interchange symmetry. > > It would be better if this reduction of terms could be done during the > recursive generation of the sum of products of pairs. Ideas > in this direction would be welcome... > > Thanks, Michael > > > -- DrMajorBob at bigfoot.com