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Re: Selecting Element from List via Pattern Matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89414] Re: [mg89396] Selecting Element from List via Pattern Matching
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Mon, 9 Jun 2008 02:26:50 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

t = List @@ Expand[(Sum[Subscript[a, i], {i, 1, 5}])^3];

Your expression works for the case given

Cases[t, Times[_, Power[Subscript[a, 2], 2]]]

{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]}

However, it will not find power terms without another factor such as

Cases[t, Times[_, Power[Subscript[a, 1], 3]]]

{}

Add a default

Cases[t, Times[_., Power[Subscript[a, 1], 3]]]

{Subscript[a, 1]^3}


Bob Hanlon

---- 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
> 



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