26 Numerics library [numerics]

26.8 Mathematical functions for floating-point types [c.math]

26.8.6 Mathematical special functions [sf.cmath] Cylindrical Bessel functions of the first kind [sf.cmath.cyl.bessel.j]

double cyl_bessel_j(double nu, double x); float cyl_bessel_jf(float nu, float x); long double cyl_bessel_jl(long double nu, long double x);
Effects: These functions compute the cylindrical Bessel functions of the first kind of their respective arguments nu and x.
where ν is nu and x is x.
Remarks: The effect of calling each of these functions is implementation-defined if nu >= 128.