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MathGroup Archive 2007

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Re: monomials in Graded Lexicographic Order and associated

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73810] Re: [mg73787] monomials in Graded Lexicographic Order and associated
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Thu, 1 Mar 2007 06:10:23 -0500 (EST)
  • References: <200702280937.EAA24658@smc.vnet.net>

er wrote:
> hi,
> 
> i just want to share my code below and ask for any suggestion to speed
> up the function FactorialList below,
> which takes up about as much time to complete as PowerList, mostly due
> to function aux. storing a table of values to avoid repeated
> computation seems to be the easiest solution. however, i'm hoping to
> avoid that, perhaps by exploiting the particular GLO structure.
> thanks.
> 
> here's the usage i'm interested in: "PowerList[GDO,max,{x1,...,xD}]
> returns { {{x1\^p1*...*xD^pD:|p|=q},q=0,...,max } where |p|=p1+...+pD;
> FactorList[GDO,D,max] returns the corresponding mv-factorial terms:
> { {p1!*...*pD!:|p|=q},q=0,...,max }", e.g.
> 
> In[1] := PowerList[GLO, 2, {a, b, c}]
> FactorialList[GLO, d, 2]
> Out[2] = {{1}, {a, b, c}, {a ^ 2, a b, a c, b^ 2, b c, c^ 2}}
> Out[2] = {{1}, {1, 1, 1}, {2, 1, 2, 1, 1, 2}}
> 
> code:
> GLO/:PowerList[GLO,0,vars_]:={{1}};
> GLO/:PowerList[GLO,p_,vars_]:=With[{rev=Reverse[vars]}, Join[{{1}},
> Flatten/@Map[Reverse,           NestList[rev*Flatten/
> @foldList[#]&,List/@rev,p-1],2]] ];
> GLO/: FactorialList[GLO,d_,p_]:=Map[Times@@Factorial[#]&,aux[d,p],{2}];

This code is missing some parts, for example the definitions of foldList 
and aux.

Anyway, I'd instead use a certain internal function. The idea is

(1) Form sums of powers of the sum of the variables.
(2) Form a sparse distributed representation of said sum with respect to 
an appropriate term order.
(3) Generate both lists in one go from that representation.

powerList[n_Integer, vars_List] /; n>=0 := Module[
   {terms=Apply[Plus,vars], dtl, newv, plist, flist},
   dtl = Internal`DistributedTermsList[
     Sum[terms^j,{j,0,n}], Reverse[vars],
	MonomialOrder->DegreeLexicographic];
   newv = Last[dtl];
   dtl = Split[Reverse[First[dtl]],
     Total[#1[[1]]]==Total[#2[[1]]]&];
   plist = Map[Apply[Times,newv^#[[1]]]&, dtl, {2}];
   flist = Map[Apply[Times,Factorial[#[[1]]]]&, dtl, {2}];
   {plist, flist}
   ]

In[4]:= InputForm[powerList[2, {a,b,c}]]

Out[4]//InputForm=
{{{1}, {a, b, c}, {a^2, a*b, b^2, a*c, b*c, c^2}},
  {{1}, {1, 1, 1}, {2, 1, 2, 1, 1, 2}}}

To give some idea of speed or lack thereof, here is how it handles a 
list of eight variables, to power 8.

In[6]:= Timing[{pl88,fl88} = powerList[8,vars];]
Out[6]= {0.920057 Second, Null}

In[7]:= LeafCount[pl88]
Out[7]= 163824

In[8]:= LeafCount[fl88]
Out[8]= 12880


Daniel Lichtblau
Wolfram Research





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