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MathGroup Archive 1996

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iterators : Goldberg's problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4040] iterators : Goldberg's problem
  • From: Vandemoortele CC Group R&D Center <wouter.meeussen.vdmcc at vandemoortele.be>
  • Date: Tue, 28 May 1996 01:46:32 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Re: in response to [mg3965]:
After a weekend's thought, maybe a better idea:

Creating a set of k indices as iterators for Table or Sum,
subject to the condition that their sum be equal to n can be realised by :

index[k_,n_]:=	Sequence@@
		ReplacePart[t=Transpose[
			{Array[a,k],
			 Table[0,{k}],
			 FoldList[#-#2&,n,Array[a,k-1]]}
		], t[[k,3]],{k,2}]  

it produces a sequence of iterators :

index[3,5]

Sequence[{a[1], 0, 5}, {a[2], 0, 5 - a[1]}, {a[3], 5 - a[1] - a[2], 5 - a[1]
- a[2]}]

The generalised Goldberg problem needs some further input:

Times@@({x,y,z}^Array[a,3])//InputForm
	x^a[1]*y^a[2]*z^a[3]

next we make a Sum over the previous expression, and wrap an 'Evaluate'
round the iterator:

Sum[Times@@({x,y,z}^Array[a,3]),Evaluate[index[3,5]]]//InputForm
x^5 + x^4*y + x^3*y^2 + x^2*y^3 + x*y^4 + y^5 + x^4*z + x^3*y*z + x^2*y^2*z
+ x*y^3*z +  y^4*z + x^3*z^2 + x^2*y*z^2 + x*y^2*z^2 + y^3*z^2 + x^2*z^3 +
x*y*z^3 + y^2*z^3 +  x*z^4 +  y*z^4 + z^5

when we further include the Multinomial :

Multinomial[Sequence@@Array[a,3]]
Multinomial[a[1], a[2], a[3]]

We end up with what Golderg wanted:

Sum[Times@@({x,y,z}^Array[a,3]) Multinomial[Sequence@@Array[a,3]],
					Evaluate[index[3,5]]]//InputForm

x^5 + 5*x^4*y + 10*x^3*y^2 + 10*x^2*y^3 + 5*x*y^4 + y^5 + 5*x^4*z + 20*x^3*y*z +
  30*x^2*y^2*z + 20*x*y^3*z + 5*y^4*z + 10*x^3*z^2 + 30*x^2*y*z^2 +
30*x*y^2*z^2 + 
  10*y^3*z^2 + 10*x^2*z^3 + 20*x*y*z^3 + 10*y^2*z^3 + 5*x*z^4 + 5*y*z^4 + z^5


Too bad that the implicit variables a[..] need definition inside the
index[_,_] function,
so that we are not at liberty to choose a name for the iterators.
The name t also lingers outside the definition of 'index[ , ]'. I don't know
how to get rid of it and still keep a one-liner (maybe with 'Module' or
with' t=.'?).

Wouter.


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