Ei(x)=-Integral from -x to infinity of e^(-t)/t dt
From specialfunctionswiki
Theorem
The following formula holds: $$\mathrm{Ei}(-x) = -\displaystyle\int_x^{\infty} \dfrac{e^{-t}}{t} \mathrm{d}t,$$ where $\mathrm{Ei}$ denotes the exponential integral Ei and $e^{-t}$ denotes the exponential.
