std::cyl_bessel_i,std::cyl_bessel_if,std::cyl_bessel_il (3) - Linux Manuals

std::cyl_bessel_i,std::cyl_bessel_if,std::cyl_bessel_il: std::cyl_bessel_i,std::cyl_bessel_if,std::cyl_bessel_il


std::cyl_bessel_i,std::cyl_bessel_if,std::cyl_bessel_il - std::cyl_bessel_i,std::cyl_bessel_if,std::cyl_bessel_il


double cyl_bessel_i( double ν, double x );
float cyl_bessel_if( float ν, float x ); (1) (since C++17)
long double cyl_bessel_il( long double ν, long double x );
Promoted cyl_bessel_i( Arithmetic ν, Arithmetic x ); (2) (since C++17)

1) Computes the regular_modified_cylindrical_Bessel_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 regular modified cylindrical Bessel function of ν and x, that is I
ν(x) = Σ∞


(for x≥0), is returned.

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


// Run this code

  #include <cmath>
  #include <iostream>
  int main()
      // spot check for ν == 0
      double x = 1.2345;
      std::cout << "I_0(" << x << ") = " << std::cyl_bessel_i(0, x) << '\n';

      // series expansion for I_0
      double fct = 1;
      double sum = 0;
      for(int k = 0; k < 5; fct*=++k) {
          sum += std::pow((x/2),2*k) / std::pow(fct,2);
          std::cout << "sum = " << sum << '\n';


  I_0(1.2345) = 1.41886
  sum = 1
  sum = 1.381
  sum = 1.41729
  sum = 1.41882
  sum = 1.41886

External links

Weisstein,_Eric_W._"Modified_Bessel_Function_of_the_First_Kind." From MathWorld--A Wolfram Web Resource.

See also

cyl_bessel_jl cylindrical Bessel functions (of the first kind)