Definition:Exponential Integral Function/Formulation 2

Definition
The exponential integral function is the real function $\Ei: \R_{>0} \to \R$ defined as:


 * $\map \Ei x = \PV_{t \mathop \to -\infty}^{t \mathop = x} \frac {e^t} t \rd t$

where $\PV$ denotes the Cauchy principal value.