The way I interpret this (as does
Maple), is that the total of the series is squared.
For example, setting a=2, and having n from 0 to 5, the sum of the series is 78. Squaring of course gives 6084.
You don't mean to square (n+a^n) do you, n^2+2na^n+a^2n? If so, the sum of the series (once again setting a=2, and having n go from 0 to 5) is 15076.
Perhaps if you could tell us what exactly you were doing we could better help you?
ILC
(I've been up for twenty hours and math has never been my specialty, get another opinion

)