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