Function bessel_j0 

pub fn bessel_j0(x: f64) -> f64

The Bessel function of the first kind of order zero, $J_{0}(x) = \frac{1}{\pi} \int_{0}^{\pi} \cos(x \sin\theta), d\theta$, by the midpoint rule on that periodic integrand (exponentially convergent: $64 + 2|x|$ points give the machine precision), as for the correlation $J_{0}(2\pi f_{D} \tau)$ of Jakes’ fading.