Re: How to select terms wanted in a series

*To*: mathgroup at smc.vnet.net*Subject*: [mg85943] Re: [mg85916] How to select terms wanted in a series*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Thu, 28 Feb 2008 02:52:10 -0500 (EST)*References*: <200802270933.EAA16377@smc.vnet.net>

On 27 Feb 2008, at 10:33, Barrow wrote: > Dear all, > > I have an expression, which is a sum of large number of terms, e.g. > > expression = x + 2x*y - x*y*z + u*x*y + 3u^2 > > And I want to select out terms with the factor (x*y). e.g. > > The terms in "expression" that I want are 2x*y, -x*y*z, u*x*y > > My first step is, > > allterms = Apply[List, expression] > > Then, I don't know how to extract the terms I want, I use > > Select[allterms, (# == x*y*_)&] > > But It doesn't work. > > Anyone has some good ideas? any help will be appreciated much! > Sincerely Barrow > Cases[expression, _?( ! FreeQ[#1, x*y] & )] {2*x*y, u*x*y, (-x)*y*z} or Select[List @@ expression, Not[FreeQ[#, x*y]] &] {2 x y, u x y, -x y z} or even x*y*List @@ (GroebnerBasis`DistributedTermsList[expression, {x, y}] [[1, 1, 2]]) {2 x y, u x y, -x y z} and many more. Andrzej Kozlowski

**References**:**How to select terms wanted in a series***From:*Barrow <GRseminar@gmail.com>