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.

x (input, double): The argument x of the function.; Constraint: x>0.0
$E1 (output, double)$: The approximation of the formula $E_1(x)=\int_x^\infty \frac{e^{-u}}udu , x>0$

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