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Re: Selecting Element from List via Pattern Matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89430] Re: [mg89396] Selecting Element from List via Pattern Matching
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Mon, 9 Jun 2008 02:29:51 -0400 (EDT)
  • References: <200806080632.CAA02645@smc.vnet.net>

On 8 Jun 2008, at 15:32, immanuel.bloch at googlemail.com wrote:

> Hello,
>
> I am trying to extract from a Power Series all elements with a
> certain exponent, e.g.
> let us say I have generated a list via
>
> t = Apply[List, Expand[(Sum[Subscript[a, i], {i, 1, 5}])^3]]
>
> How can I then extract all terms with e.g. a_2^2 in it or any other
> exponent?
>
> I have tried something like
>
> Cases[t, Times[_, Power[Subscript[a,2],2]]]
>
> but this somehow only works with the exponent n=1 and no others...
>
> Does anybody know a solution to this pattern matching problem?
>
> Thanks!
> Immanuel
>

But what's wrong with simply doing this:

  Expand[Coefficient[Sum[Subscript[a, i], {i, 1, 5}]^3, Subscript[a,  
2]^2]*
      Subscript[a, 2]^2]

3*Subscript[a, 1]*Subscript[a, 2]^2 +
    3*Subscript[a, 3]*Subscript[a, 2]^2 +
    3*Subscript[a, 4]*Subscript[a, 2]^2 +
    3*Subscript[a, 5]*Subscript[a, 2]^2

???


Andrzej Kozlowski



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