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.

For more information please review the s13aac function in the NAG document.

Parameters:

x (input, double)

The argument x of the function.

Constraint: x>0.0

\[E1 (output, double)\]

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