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Re: Selecting Element from List via Pattern Matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89429] Re: Selecting Element from List via Pattern Matching
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Mon, 9 Jun 2008 02:29:40 -0400 (EDT)
  • References: <g2fugh$2kq$1@smc.vnet.net>

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?
> 

It is not completely clear to me what you mean.  Evaluating

Cases[t, _ Subscript[a, 2]^2]

gives

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

Isn't this what you expected?


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