Let $I_n=\int_{0}^{1}e^{-y}y^n\ dy$, where $n$ is non-negative integer. Find $\sum_{n=1}^{\infty}\frac{I_n}{n!}.$
I first solved $I_n$ and obtained $$I_n=-\frac{1}{e}+nI_{n-1} \\ \hspace{35mm} =-\frac{1}{e}+n(-\frac{1}{e}+(n-1)I_{n-2}) \\ \hspace{72.7mm}=-\frac{1}{e}-n\frac{1}{e}-n(n-1)\frac{1}{e}-n(n-1)\frac{1}{e}+...+n!(I_1)$$
Since $$I_1=1-\frac{2}{e}\\ \therefore \frac{I_n}{n!}=-\frac{1}{e}(\frac{1}{n!}+\frac{1}{(n-1)!}+\frac{1}{(n-2)!}...+\frac{1}{2})+(1-\frac{2}{e})$$
I don't know what to do after this, because taking $e=2+1/2!+1/3!+1/4!+...+1/n!$ gives the value of $\frac{I_n}{n!}$ to be $0$.
How do I solve this?
How about $$ \sum\limits_{n = 1}^\infty {\frac{{I_n }}{{n!}}} = \sum\limits_{n = 1}^\infty {\frac{1}{{n!}}\int_0^1 {e^{ - y} y^n dy} } = \int_0^1 {e^{ - y} \sum\limits_{n = 1}^\infty {\frac{{y^n }}{{n!}}} dy} = \int_0^1 {e^{ - y} (e^y - 1)dy} = e^{ - 1} \,? $$