Re: Keeping terms of certain order in expand command

*To*: mathgroup at smc.vnet.net*Subject*: [mg31609] Re: Keeping terms of certain order in expand command*From*: Tom Burton <tburton at cts.com>*Date*: Fri, 16 Nov 2001 06:38:16 -0500 (EST)*Organization*: Brahea Consulting*References*: <9t0435$3o6$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hello, Here is a straightforward implementation of your words. First, form the expression to be expanded: expr1 = Plus @@ Table[Subscript[T, i], {i, 3, 8}] Then expand: expr2 = Expand[expr1^5] Now it's just a matter of slightly rewriting your words. Splitting off the coefficient c, each term is converted from a product to a sum (inner Plus@@), then the order expression is formed, divided by 2, and then checked whether it is an integer or not; i.e., whether the order expression itself is even or not. Even terms are retained. Finally, the result of Cases, a list, is converted to a sum (outer Plus@@). expr3 = Plus @@ Cases[expr2, c_Integer*term_ /; IntegerQ[ (Plus @@ term /. {Subscript[T, i_] :> i, Subscript[T, i_]^j_ :> i*j})/2]] There may be a clever, much briefer solution, but this is straightforward. Tom Burton On Thu, 15 Nov 2001 10:06:29 +0000 (UTC), in comp.soft-sys.math.mathematica you wrote: >Hello All, > >I am tring to write a program that will do the following but I cant seem to >get it right. > >I would like to keep terms of the expansion below that have >even order where order is defined as follows: > > >The order of T_x is x*1 x where 1 is the exponent of the term, > _ (underscore) is for sub, and ^ is for raised to the power. > > >the order of T_x * T_y = x*1 + y*1 >the order of ((T_x)^i) * ((T_y)^j)) * ((T_z)^k) is > x*i + y*j + z*k > >Expand[(T_3 + T_4 + T_5+ T_6+ T_7+T_8)^5] > >Thanks A lot, >Chris