std::expint, std::expintf, std::expintl

double      expint( double arg );
double      expint( float arg );
double      expint( long double arg );
float       expintf( float arg );
long double expintl( long double arg );

double      expint( IntegralType arg );

Parameters

Return value

Error handling

Errors may be reported as specified in math_errhandling.

• If the argument is NaN, NaN is returned and domain error is not reported
• If the argument is ±0, -∞ is returned

Notes

Implementations that do not support C++17, but support ISO 29124:2010, provide this function if __STDCPP_MATH_SPEC_FUNCS__ is defined by the implementation to a value at least 201003L and if the user defines __STDCPP_WANT_MATH_SPEC_FUNCS__ before including any standard library headers.

Implementations that do not support ISO 29124:2010 but support TR 19768:2007 (TR1), provide this function in the header tr1/cmath and namespace std::tr1.

An implementation of this function is also available in boost.math.

Example

#include <cmath>
#include <iostream>
int main()
{
    std::cout << "Ei(0) = " << std::expint(0) << '\n'
              << "Ei(1) = " << std::expint(1) << '\n'
              << "Gompetz constant = " << -std::exp(1)*std::expint(-1) << '\n';
}

Ei(0) = -inf
Ei(1) = 1.89512
Gompetz constant = 0.596347

External links

Weisstein, Eric W. "Exponential Integral." From MathWorld--A Wolfram Web Resource.