# How to find sum of the power series $\sum_{n=1}^{\infty} x^2 e^{-nx}$?

I got this power series $$\sum_{n=1}^{\infty} x^2 e^{-nx}$$ And i need to prove uniform convergence when $x \in[0;1]$ and find this sum. I have proven uniform convergence, but i have no idea how to find this sum. It's clear, that $\sum_{n=1}^{\infty} x^2 e^{-nx}$ is a geometric series $\frac{x^2 e^{-x}}{1-e^{-x}}$. Is it correct that $\sum_{n=1}^{\infty} x^2 e^{-nx}$ equals $\int_0^1 \frac{x^2 e^{-x}}{1-e^{-x}}$?

• Do you need the value of the series or the value of an integral over this series? You have not clearly stated why you would need the $\int_0^1$ part in your question. – Formyer Oct 9 '17 at 2:05
• @Formyer i need find value of the series – Shmuser Oct 9 '17 at 2:06
• Seems to me you already have all the answers (convergence, geometric serie, the value of the sum).What is it, you need exactly? – zwim Oct 9 '17 at 2:08
• @Formyer I thought that value of the series and the value of an integral over this series are the same, aren't they? – Shmuser Oct 9 '17 at 2:09
• @user401689 No. The series depends on $x$ but the integral is a specific number, so even without computing either of them, you can see they can't be the same. I'm not sure why you even integrated it, in the first place. – Jam Oct 9 '17 at 2:14

$$\sum_{n=1}^\infty x^2e^{-nx}=x^2\frac{e^{-x}}{1-e^{-x}}.$$

You seem to conflate a few different concepts here: For one,

$$\sum_{n=1}^\infty x^2e^{-nx}=\lim_{k\rightarrow\infty}\sum_{n=1}^k x^2e^{-nx}$$

(a series is the limit of its partial sums) is the definition of a series.

Then, you have some "similarities" between series and integrals that do not factor into this problem: For starters, the Riemann integral approach:

$$\sum_{i=1}^N f(x_i)\Delta x_i\rightarrow\int_a^bf(x)\text dx$$

for a partition $a=x_1<...<x_N=b$ of an interval $[a,b]$, with $\Delta x_i=x_{i+1}-x_i$. The limit converges for any sequence of partitions as long as the mesh size $\max_{i=1,...,N-1}(x_{i+1}-x_i)$ converges towards zero.

Then, there is the integrability criterion / integral test that lets you determine whether a series converges at all by comparing it to the integral over its summation sequence. That only holds for non-negative monotonely decreasing functions, though, and is a comparison, not an equality.

Again, none of this has anything to do with the answer to your question, but might clear up some concepts you seem to have difficulty grasping.