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Re: Selecting Element from List via Pattern Matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89461] Re: Selecting Element from List via Pattern Matching
  • From: Tom <th.meurer at gmail.com>
  • Date: Tue, 10 Jun 2008 03:40:32 -0400 (EDT)
  • References: <g2fugh$2kq$1@smc.vnet.net>

On Jun 8, 8:33 am, immanuel.bl... 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


Hi, actually I do not understand why you convert the series into a
list. If you stick with the series just use

ex[f_, c_] := Coefficient[f /. {c -> s}, s]
ex[t, Subscript[a, 2]^2]

which does the job.
Thomas


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