#include <cmath>

 double cyl_bessel_k(double nu, double x)  // C++17

Liefert Wert der modifizierten Bessel-Funktion 2. Art $K_\nu(x) = \int_0^\infty e^{-x \cosh t} \cosh (\nu t) dt = \frac{\pi}{2} \frac{I_{-\nu}(x)-I_\nu (x)}{\sin (\nu\pi)}$ für $x \geq 0$.

Rückgabewert: $J_\nu(x)$.

cyl_bessel_i(), cyl_bessel_j(), cyl_neumann().

cyl_bessel_k.cpp

#include <cmath>
#include <iostream>
 
int main()
{
  std::cout << "# x n=0 n=1 n=2\n";
 
  for (int i = 0; i <= 1000; ++i)
  {
    double x = 0.01*i;
    std::cout << x 
      << '\t' << std::cyl_bessel_k(0, x) 
      << '\t' << std::cyl_bessel_k(1, x) 
      << '\t' << std::cyl_bessel_k(2, x) 
      << '\n';
  }
}