\begin{definition}[Definition:Exponential Integral Function]
The '''exponential integral function''' is the real function $\Ei: \R_{>0} \to \R$ defined as:
:$\map \Ei x = \ds \int_{t \mathop = x}^{t \mathop \to +\infty} \frac {e^{-t} } t \rd t$
\end{definition}