std::cyl_be...yl_bessel_jl(3)

🌸 NAME 🌸

Okay so this page is all about std::cyl_bessel_j, std::cyl_bessel_jf, and std::cyl_bessel_jl — the C++ Standard Library’s cylindrical Bessel functions of the first kind. The manual name is literally that trio, bestie, so we’re folding it in right here instead of copying the header line. Slay. 💅

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✨ Synopsis ✨

It lives in the header <cmath>:

Defined in header <cmath>

float       cyl_bessel_j ( float nu, float x );               (since C++17)
double      cyl_bessel_j ( double nu, double x );             (until C++23)
long double cyl_bessel_j ( long double nu, long double x );

/* floating-point-type */ cy_bessel_j( /* floating-point-type */ nu,
                                        /* floating-point-type */ x );  (1) (since C++23)

float       cyl_bessel_jf( float nu, float x );               (2) (since C++17)
long double cyl_bessel_jl( long double nu, long double x );   (3) (since C++17)

Additional overloads also live in <cmath>:

Additional overloads
Defined in header <cmath>

template< class Arithmetic1, class Arithmetic2 >
/* common-floating-point-type */
cyl_bessel_j( Arithmetic1 nu, Arithmetic2 x );                 (A) (since C++17)

1. Computes the cylindrical Bessel function of the first kind of nu and x.

The library provides overloads of std::cyl_bessel_j for all cv-unqualified floating-point types as the type of the parameters nu and x. (since C++23)

A) Additional overloads are provided for all other combinations of arithmetic types.

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🎀 Parameters 🎀

• nu — the order of the function
• x — the argument of the function

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🦋 Return value 🦋

If no errors occur, you get the value of the cylindrical Bessel function of the first kind of nu and x, that is:

J_nu(x) = Σ_{k=0}^{∞}  (-1)^k (x/2)^(nu+2k) / (k! Γ(nu+k+1))    (for x ≥ 0)

…and that value is returned. ✨

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💅 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 nu≥128, the behavior is implementation-defined.

Heads up, bestie: that nu≥128 case is implementation-defined, so don’t act surprised if different implementations do different things. No cap. 😭

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🌈 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.

Lowkey important: the additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their first argument num1 and second argument num2:

• If num1 or num2 has type long double, then std::cyl_bessel_j(num1, num2) has the same effect as std::cyl_bessel_j(static_cast<long double>(num1), static_cast<long double>(num2)).
• Otherwise, if num1 and/or num2 has type double or an integer type, then std::cyl_bessel_j(num1, num2) has the same effect as (until C++23) std::cyl_bessel_j(static_cast<double>(num1), static_cast<double>(num2)).
• Otherwise, if num1 or num2 has type float, then std::cyl_bessel_j(num1, num2) has the same effect as std::cyl_bessel_j(static_cast<float>(num1), static_cast<float>(num2)).

And (since C++23), if num1 and num2 have arithmetic types, then std::cyl_bessel_j(num1, num2) has the same effect as:

std::cyl_bessel_j(static_cast</* common-floating-point-type */>(num1),
                  static_cast</* common-floating-point-type */>(num2))

where /* common-floating-point-type */ is the floating-point type with the greatest floating-point conversion rank and greatest floating-point conversion subrank between the types of num1 and num2. Arguments of integer type are considered to have the same floating-point conversion rank as double.

If no such floating-point type with the greatest rank and subrank exists, then overload resolution does not result in a usable candidate from the overloads provided.

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🍓 Example 🍓

Run this code:

#include <cmath>
#include <iostream>

int main()
{
    // spot check for nu == 0
    const 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

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👑 See also 👑

• cyl_bessel_i, cyl_bessel_if, cyl_bessel_il — regular modified cylindrical Bessel functions, (C++17) (function) (C++17) (C++17)

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⭐ External links ⭐

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

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That’s the whole page, bestie — you ate. 💖 Source/version: http://cppreference.com; date: 2024.06.10. Mwah 💋🎀