std::cyl_neumann, std::cyl_neumannf, std::cyl_neumannl

double      cyl_neumann( double ν, double x );

float       cyl_neumannf( float ν, float x  );

long double cyl_neumannl( long double ν, long double x );
(1) (since C++17)
Promoted    cyl_neumann( Arithmetic ν, Arithmetic x );
(2) (since C++17)
1) Computes the cylindrical Neumann function (also known as Bessel function of the second kind or Weber function) of ν and x.
2) A set of overloads or a function template for all combinations of arguments of arithmetic type not covered by (1). If any argument has integral type, it is cast to double. If any argument is long double, then the return type Promoted is also long double, otherwise the return type is always double.


ν - the order of the function
x - the argument of the function)

Return value

If no errors occur, value of the cylindrical Neumann function (Bessel function of the second kind) of ν and x, is returned, that is N
(x) =
(where J
is std::cyl_bessel_j(ν,x)) for x≥0 and non-integer ν; for integer ν a limit is used.

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 ν>=128, the behavior is implementation-defined


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


#include <cmath>
#include <iostream>
int main()
    double pi = std::acos(-1);
    double x = 1.2345;
    // spot check for ν == 0.5
    std::cout << "N_.5(" << x << ") = " << std::cyl_neumann(.5, x) << '\n'
              << "calculated via J = " << 
             - std::cyl_bessel_j(-.5,x))/std::sin(.5*pi) << '\n';


N_.5(1.2345) = -0.236973
calculated via J = -0.236973

External links

Weisstein, Eric W. "Bessel Function of the Second Kind." From MathWorld--A Wolfram Web Resource.

See also

regular modified cylindrical Bessel functions
cylindrical Bessel functions (of the first kind)
irregular modified cylindrical Bessel functions