# Question:Select some terms from an expression

## Question:Select some terms from an expression

Maple 2015

Dear users!

I hope everyone is fine here. I have the following expression:

r*y[0, 1]+y[0, 0]+(1/6)*r*(r-1)*(1+r)*y[-1, 3]+(1/2)*r*(r-1)*y[-1, 2]+(1/120)*r*(r-1)*(r-2)*(1+r)*(2+r)*y[-2, 5]+(1/24)*r*(r-1)*(r-2)*(1+r)*y[-2, 4]+(1/720)*r*(r-1)*(r-2)*(r-3)*(r-4)*(1+r)*y[-3, 8]+(1/360)*r*(r-1)^2*(r-2)*(r-3)*(1+r)*y[-3, 7]+(1/720)*r*(r-1)*(r-2)*(r-3)*(1+r)*(2+r)*y[-3, 6]:

What is the procedure to select some terms in the above expression for example for N=2 I just want the following terms:

y[0, 0]+r*y[0, 1]+(1/2)*r*(r-1)*y[-1, 2];

and for N=3 I just want the following terms:

y[0, 0]+r*y[0, 1]+(1/2)*r*(r-1)*y[-1, 2]+(1/6)*r*(r-1)*(1+r)*y[-1, 3];

and for N=4 I want:

(1/24)*r*(r-1)*(r-2)*(1+r)*y[-2, 4]+y[0, 0]+r*y[0, 1]+(1/2)*r*(r-1)*y[-1, 2]+(1/6)*r*(r-1)*(1+r)*y[-1, 3];

and so on,

in the descending order in first suffices and ascending order in second suffices (like term having y[0,0],  y[0,1], y[-1,2], y[-1,3], y[-2,4]). I am waiting for your response. Thanks.

﻿