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Re: Symmetrizing function arguments

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  • Subject: [mg127618] Re: Symmetrizing function arguments
  • From: W Craig Carter <ccarter at MIT.EDU>
  • Date: Wed, 8 Aug 2012 03:14:04 -0400 (EDT)
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Hello Hauke,
Perhaps you are looking for the Orderless attribute?

SetAttributes[tet, Orderless]

W Craig Carter
Professor of Materials Science, MIT

On Aug 7, , at Tue Aug 7, 12 @3:01 AM, Hauke Reddmann wrote:

> I'd like to define a quasi 6j symbol which has tetrahedron
> symmetry in its 6 arguments. At the moment (I'm a n00b, still :-)
> I use a cheap hack:
> f[a_,b_] := If[a>b,F[a,b],F[b,a]];
> Bingo, f now is commutative and sorts by descending arguments,
> and NOOOOO endless loops. My, am I proud of myself :-)
> Needless to say, using this method to implement the symmetries
> of h[a_,b_,c_,d_,e_,f] is a royal pain in the backside, as you
> see with the hassle needed already for just 3 arguments...
> g[a_,b_,c_]:=If[b>c,If[a>c,If[a>b,G[a,b,c],g[b,a,c]],g[c,b,a]],g[a,c,b]];
> ...especially considering all the subcases needed when two arguments 
> are equal. 
> Surely, you can offer a more elegant way? It more or less suffices to 
> bring the largest value to position 1 and the second-largest to 2 or 3
> (assume a,b and c,d and e,f are the opponent edges of the tetrahedron),
> but optimal would be if none of the 24 tetrahedron operations gives
> a smaller lexicalic ordering even in the case of equal entries.
> P.S. No, the inbuilt 6j symbol is useless - wrong Lie group :-)
> -- 
> Hauke Reddmann <:-EX8    fc3a501 at
> Out on deck the dawn arrived
> Your grey sweater oversized
> The rooftops glimmered before our eyes

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