Hello,

the problem is to show by calculating that:

$\displaystyle \displaystyle\frac{x(1+x)}{(1-x)^3}=\sum_{i=0}^{\infty}k^2x^k$.

I know the solution has to do something formal power series. I found following pieces: link 1 and link 2 , but even those couldn't get me any further. Also WolframAlpha didn't gave me any clue, though there's some information. I'm completely stuck, which means any help is welcome. Thank you!