# 3.5.2.5.26 Exp_integral

## Definition:

$E1 = exp\_integral(x)$ evaluates

$E_1(x)=\int_x^\infty \frac{e^{-u}}udu , x>0$

The approximation is based on several Chebyshev expansions.

$E1 (output, double)$
The approximation of the formula $E_1(x)=\int_x^\infty \frac{e^{-u}}udu , x>0$