Re: Selecting subscripted variables from an expression
- To: mathgroup at smc.vnet.net
- Subject: [mg109152] Re: Selecting subscripted variables from an expression
- From: Bill Rowe <readnews at sbcglobal.net>
- Date: Wed, 14 Apr 2010 05:15:19 -0400 (EDT)
On 4/12/10 at 10:59 PM, miliotodc at rtconline.com (Dominic) wrote: >Hi guys. I have an expression containing terms with subscripted "a" >and "b" variables. Can someone help me devise an algorithm to >select from this sum, all the terms that have a particular >subscripted variable as the first term, such as all the terms that >begin with an a_1 in them. For example, if I have the expression: >myNumerator=2a_1a_2b_2 + a_2 + a_1b_2b_3 + a_3b_2_b_3 >then the routine would pick out the 2a_1a_2b_2 term and the >a_1b_2b_3 terms. >The expression below is a particular example. So I suppose the >routine might look like some version of: >Select[myNumerator, (all terms that begin with a_1 for example)] >myNumerator = Subscript[a, 1]*Subscript[b, 0]^5 - Subscript[a, >2]*Subscript[b, 0]^4*Subscript[b, 1] + Subscript[a, 3]*Subscript[b, >0]^3*Subscript[b, 1]^2 - Subscript[a, 4]*Subscript[b, >0]^2*Subscript[b, 1]^3 + Subscript[a, 5]*Subscript[b, >0]*Subscript[b, 1]^4 - Subscript[a, 6]*Subscript[b, 1]^5 - >Subscript[a, 3]*Subscript[b, 0]^4* Subscript[b, 2] + 2*Subscript[a, >4]*Subscript[b, 0]^3*Subscript[b, 1]* Subscript[b, 2] - >3*Subscript[a, 5]*Subscript[b, 0]^2*Subscript[b, 1]^2* Subscript[b, >2] + 4*Subscript[a, 6]*Subscript[b, 0]*Subscript[b, 1]^3* >Subscript[b, 2] + Subscript[a, 5]*Subscript[b, 0]^3*Subscript[b, >2]^2 - 3*Subscript[a, 6]*Subscript[b, 0]^2*Subscript[b, >1]*Subscript[b, 2]^2 - Subscript[a, 4]*Subscript[b, >0]^4*Subscript[b, 3] + 2*Subscript[a, 5]*Subscript[b, >0]^3*Subscript[b, 1]*Subscript[b, 3] - 3*Subscript[a, >6]*Subscript[b, 0]^2*Subscript[b, 1]^2*Subscript[b, 3] + >2*Subscript[a, 6]*Subscript[b, 0]^3*Subscript[b, 2]*Subscript[b, 3] >- Subscript[a, 5]*Subscript[b, 0]^4*Subscript[b, 4] + 2*Subscript[a, >6]*Subscript[b, 0]^3*Subscript[b, 1]*Subscript[b, 4] - Subscript[a, >6]*Subscript[b, 0]^4*Subscript[b, 5]; In[10]:= DeleteCases[ myNumerator, _?(FreeQ[#, Subscript[a, 1]] & )] Out[10]= Subscript[a, 1]*Subscript[b, 0]^5