Re: Cyclic generator of a cyclic expression
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- Subject: [mg131745] Re: Cyclic generator of a cyclic expression
- From: danl at wolfram.com
- Date: Fri, 27 Sep 2013 02:28:52 -0400 (EDT)
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On Thursday, September 26, 2013 2:41:29 AM UTC-5, Francisco Javier Garc=EDaCapit=E1n wrote: > Dear friends, > > Given a cyclic expression say in x,y,z like > > x^4 + y^4 + z^4 - y^2 z^2 - z^2 x^2 - x^2 y^2 > > what is your more or less elegant way to get a cyclic generator, like > > x^4 - y^2 z^2 ? > > Thank you very much. > > --- > > Francisco Javier Garc=EDa Capit=E1n > > http://garciacapitan.99on.com This may seem slightly backwards but we'll first show code to do the reverse, that is, invariantize a given expression with respect to cyclic shifting of the vvariable list. Note that this is order-dependent so we must provide the explicit list. cyclicInvariantize[expr_, vars_] := Sum[expr /. Thread[vars -> RotateRight[vars, j]], {j, Length[vars]}] Example (reverse of what you want): In[62]:= ci = cyclicInvariantize[x^4 - y^2 z^2, {x, y, z}] Out[62]= x^4 - x^2 y^2 + y^4 - x^2 z^2 - y^2 z^2 + z^4 Now we use this as follows. For each monomial in a given input, invariantize it. Then for each first occurrence of an invariantized subexpression, choose the monomial that gave rise to it. Sum the chosen monomials. In[80]:= cyclicGenerator[expr_Plus, vars_] := Module[ {mtab = List @@ expr, seen, reptab}, invars = cyclicInvariantize[#, vars] & /@ mtab; reptab = Table[If[TrueQ[seen[invar]], False, seen[invar] = True; True], {invar, invars}]; Total[Pick[mtab, reptab]] ] Your example: In[81]:= cyclicGenerator[ci, {x, y, z}] Out[81]= x^4 - x^2 y^2 Daniel Lichtblau Wolfram Research