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Showing content from https://en.cppreference.com/w/cpp/language/../numeric/special_functions/cyl_bessel_i.html below:

std::cyl_bessel_i, std::cyl_bessel_if, std::cyl_bessel_il - cppreference.com

/* floating-point-type */ cyl_bessel_i( /* floating-point-type */ nu,
                                        /* floating-point-type */ x );

float       cyl_bessel_if( float nu, float x );

long double cyl_bessel_il( long double nu, long double x );

Computes the

of

and

.

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

If no errors occur, value of the regular modified cylindrical Bessel function of

and

, that is

(for

), is returned.

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.

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.

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_i(num1, num2) has the same effect as std::cyl_bessel_i(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_i(num1, num2) has the same effect as std::cyl_bessel_i(static_cast<double>(num1),
                    static_cast<double>(num2)).
• Otherwise, if num1 or num2 has type float, then std::cyl_bessel_i(num1, num2) has the same effect as std::cyl_bessel_i(static_cast<float>(num1),
                    static_cast<float>(num2)).

If num1 and num2 have arithmetic types, then std::cyl_bessel_i(num1, num2) has the same effect as std::cyl_bessel_i(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.

#include <cmath>
#include <iostream>
 
int main()
{
    // spot check for nu == 0
    const 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';
    }
}

Output:

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

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