题目内容：

请问如何对数列x^n/(n*n!)求和（n趋于无穷大）,如1 + x + x^2/（2!×2）+ x^3/（3!×3）...,
如题.都困扰很久了:(
Actually there is no elementary derivative.Have to use exponential integral Ei(x) to define the complex elements.Still thanks.