[?] Increment in multiple sum indices?

[shahzeb1618 (Shahzeb Kamal)]

Hello,
I am trying to write nested sums of the following form:

But i need the index j to run from 0 with an increment as follows:

any suggestions? Thanks
[1]: http://community.wolfram.com//c/portal/getImageAttachment?filename=2631question.jpg&userId=1221116
[2]: http://community.wolfram.com//c/portal/getImageAttachment?filename=answer.jpg&userId=1221116

[gianlucagorni]

You can use Fold:

With[{k = 2},
 Fold[Sum[#1, {Subscript[a, #2], 0, n}] &, Sum[a[i] x^i, {i, 0, n}], 
  Range[0, k]]]

[shahzeb1618 (Shahzeb Kamal)]

Thanks Solved!