# std::cyl_bessel_j, std::cyl_bessel_jf, std::cyl_bessel_jl

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< cpp‎ | numeric‎ | special math

double cyl_bessel_j( double ν, double x ); float cyl_bessel_jf( float ν, float x ); long double cyl_bessel_jl( long double ν, long double x ); | (1) | (since C++17)
Promoted cyl_bessel_j( Arithmetic ν, Arithmetic x ); | (2) | (since C++17)

1) Computes the cylindrical Bessel function of the first kind 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.

### Parameters

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

### Return value

If no errors occur, value of the cylindrical Bessel function of the first kind of `ν` and `x`, that is J
ν(x) = Σ∞
k=0

(-1)k (x/2)ν+2k
k!Γ(ν+k+1)

(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

### 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

Run this code

    #include <cmath>
    #include <iostream>
    int main()
    {
        // spot check for ν == 0
        double x = 1.2345;
        std::cout << "J_0(" << x << ") = " << std::cyl_bessel_j(0, x) << '\n';

        // series expansion for J_0
        double fct = 1;
        double sum = 0;
        for(int k = 0; k < 6; fct*=++k) {
            sum += std::pow(-1, k)*std::pow((x/2),2*k) / std::pow(fct,2);
            std::cout << "sum = " << sum << '\n';
        }
    }

Output:

    J_0(1.2345) = 0.653792
    sum = 1
    sum = 0.619002
    sum = 0.655292
    sum = 0.653756
    sum = 0.653793
    sum = 0.653792

### External links

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

### See also

cyl_bessel_icyl_bessel_ifcyl_bessel_il (C++17)(C++17)(C++17) | regular modified cylindrical Bessel functions (function)

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