expint()

#include <cmath>

 double expint(double x)  // C++17

Liefert Wert der Integralexponentialfunktion $Ei(x) = -\int_{-x}^\infty \frac{e^{-t}}{t}dt$.

Rückgabewert: $Ei(x)$.

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expint.cpp

#include <cmath>
#include <iostream>
 
int main()
{
  for (int i = -500; i <= 500; ++i)
  {
    double x = 0.01*i;
    std::cout << x << '\t' << std::expint(x) << '\n';
  }
}