$$E_n\left(x\right)=\frac{e^{-x}}{n+x-\frac{n}{2+n+x-\frac{2\left(n+1\right)}{4+n+x-\frac{3\left(n+2\right)}{6+n+x-\frac{4\left(n+3\right)}{8+n+x-\dots }}}}}$$