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Re: compositions with the restricted set of integers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg105168] Re: [mg105143] compositions with the restricted set of integers
  • From: michael partensky <partensky at gmail.com>
  • Date: Mon, 23 Nov 2009 06:51:37 -0500 (EST)
  • References: <200911221110.GAA10476@smc.vnet.net>

Thanks, Daniel.
Although it was meant as an exercise on Integer partitioning for the
students, your suggestion adds an important  twist to this endeavor. And it
really flies!

Best.
Michael

On Sun, Nov 22, 2009 at 9:03 PM, <danl at wolfram.com> wrote:

> >  IntegerPartitions has a useful option restricting the values of
> > partitions.
> > For example, IntegerPartitions[6,2,{1,2,3,4,5,6}] . Does it exist for
> > Compositions?
> >
> > What I am looking for is a substitute for  a lengthy and inefficient
> > approach described below. We through n dice.  How many outcomes
> > *outs*correspond to the face total equal
> > *sum*?
> > I am aware about the recursive approach, but prefer to describe it in
> > terms
> > of integer partitioning (or compositions).
> > *Here is my intellectually insulting solution:*
> >
> > outs[sum_, thr_] :=
> >   Length[Flatten[
> >     Permutations /@
> >      Cases[IntegerPartitions[sum, thr, {1, 2, 3, 4, 5, 6}],
> >       Table[_, {thr}]], 1]];
> >
> > Mapping with  permutations seems especially silly.
> > Is there a version of Compositions with the similar restrictive option
> > ({1,
> > 2, 3, 4, 5, 6})?
> > I would appreciate any other suggestions as well.
> >
> > Thanks
> > Michael
>
> I'm not sure this addresses your question, but a good way to do such
> computations is via generating functions.
>
> If each die has m outcomes, numbering 1-m, and we have some give number of
> throws, then the number of ways to get a given sum can be found as:
>
> countsums[sum_, throws_, m_] :=
>  SeriesCoefficient[(z*(1 - z^m)/(1 - z))^throws, {z, 0, sum}]
>
> This example agrees with your outs[30,8].
>
> In[7]:= countsums[30, 8, 6]
> Out[7]= 125588
>
> This example does not go into recursion, run out of memory, or in other
> ways cause trouble.
>
> In[8]:= countsums[300, 80, 6]
> Out[8]= 1986180404791119810432501428481165305715190860615069661053632
>
> Daniel Lichtblau
> Wolfram Research
>
>
>



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