26 Numerics library [numerics]

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

26.8.6 Mathematical special functions [sf.cmath] Regular modified cylindrical Bessel functions [sf.cmath.cyl.bessel.i]

double cyl_bessel_i(double nu, double x); float cyl_bessel_if(float nu, float x); long double cyl_bessel_il(long double nu, long double x);
Effects: These functions compute the regular modified cylindrical Bessel functions 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.