Definition:Exponential Integral Function/Formulation 1

Definition
The exponential integral function is the real function $E_1: \R_{>0} \to \R$ defined as:


 * $\map {E_1} x = \ds \int_{t \mathop = x}^{t \mathop \to +\infty} \frac {e^{-t} } t \rd t$