Does anyone know of a closed formula for the function $f_k(x)=\sum_{n=1}^{\infty}{n^k x^n}$ ? That is, the generating function of the sequence $1^k,2^k,3^k...$.

It is not hard to see that $f_k(x)=\frac{P(x)}{(1-x)^{k+1}}$, where $P(x)$ is a monic polynomial of degree $k$ (this follows from the identity $f_k(x)=x\cdot f'_{k-1}(x)$ ). A closed formula for the coefficients of $P(x)$ would be very helpful.

Thanks for your attention and efforts!

Lior