# Computing:$\sum_{n=0}^\infty\frac{3^n}{n!(n+3)}$

I'm learning the subject Power Series and I can't figure out how to find the sum of the series $$\sum_{n=0}^\infty\frac{3^n}{n!(n+3)}$$.

I know that the power series $\sum_{n=0}^\infty\frac{3^n}{n!(n+3)}x^n$ is convergence for every $x$, and uniform convergence for every $[-r, r], r>0$ because $\lim\limits_{n \to\infty}|\frac{a_n}{a_{n+1}}|=\infty$. I don't know how to proceed, I think I should use the information that this power series convergence uniformly and then somehow Integrating/differentiating this Power Series.

Thanks!

• What do you want to prove? – mathcounterexamples.net Jan 14 '18 at 17:07
• I don't get what's unclear. They OP is asking how to find the sum (the value) of the series. – G Tony Jacobs Jan 14 '18 at 18:18

## 2 Answers

$$\sum_{n=0}^{+\infty}\frac{3^n}{n!\left(n+3\right)}=\frac{1}{27}\sum_{n=0}^{+\infty}\frac{1}{n!}\frac{3^{n+3}}{n+3}$$ And $$\frac{3^{n+3}}{n+3}=\int_{0}^{3}x^{n+2}\text{d}x$$ Hence it becomes ( using uniform convergence on every compact of $\mathbb{R}$ especially $\left[0,3\right]$ ) $$\sum_{n=0}^{+\infty}\frac{3^n}{n!\left(n+3\right)}=\frac{1}{27}\sum_{n=0}^{+\infty}\frac{1}{n!}\int_{0}^{3}x^{n+2}\text{d}x=\frac{1}{27}\int_{0}^{3}\sum_{n=0}^{+\infty}\frac{1}{n!}x^{n+2}\text{d}x$$ Then $$\sum_{n=0}^{+\infty}\frac{3^n}{n!\left(n+3\right)}=\frac{1}{27}\int_{0}^{3}x^2e^{x}\text{d}x$$ Hence you conlude ( integrating by parts ) $$\sum_{n=0}^{+\infty}\frac{3^n}{n!\left(n+3\right)}=\frac{1}{27}\left(5e^3-2\right)$$

You can merely use the differentiation of power series as follows

$$\sum_{n=0}^\infty\frac{3^n}{n!(n+3)}= \sum_{n=0}^\infty\frac{(n+2)(n+1)3^n}{(n+3)!}=\sum_{n=3}^\infty\frac{(n-1)(n-2)3^{n-3}}{n!}\\= \frac{d^2}{dx^2}\left( \sum_{n=1}^\infty\frac{x^{n-1}}{n!}\right)\Bigg|_{x=3}= \frac{d^2}{dx^2}\left( -\frac{1}{x}+\frac1x\sum_{n=0}^\infty\frac{x^{n}}{n!}\right)\Bigg|_{x=3}\\=\frac{d^2}{dx^2}\left(-\frac{1}{x} +\frac{e^{x}}{x}\right)\Bigg|_{x=3}= \left(-\frac{2}{x^3} +\frac{e^{x}(x^2-2x+2)}{x^3}\right)\Bigg|_{x=3} \\ = \color{red}{\frac{1}{27}\left(5e^3-2\right)}$$

• Shouldn't the second sum be $\sum\frac{(n+2)(n+1)3^n}{(n+3)!}$? – Lord Shark the Unknown Jan 14 '18 at 17:34
• @LordSharktheUnknown. thanks I correct it now – Guy Fsone Jan 14 '18 at 17:35