double cyl_bessel_k(double nu, double x); float cyl_bessel_kf(float nu, float x); long double cyl_bessel_kl(long double nu, long double x);
Kν(x)=(π/2)iν+1(Jν(ix)+iNν(ix))=⎧⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪⎨⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪⎩π2I−ν(x)−Iν(x)sinνπ,for x≥0 and non-integral νπ2limμ→νI−μ(x)−Iμ(x)sinμπ,for x≥0 and integral ν
where ν is nu and x is x.