After repeatedly having to add 3rd party libraries only for the these functions or having to implement them myself quick and dirty based on numerical integration, I suggest add the Bessel functions of the first and second kind to the math module. These functions tend to appear a lot (but not restricted to) when evaluating systems that are defined on cylindrical geometries, restrictions or approximations (like the proximate solution to Kepler's equation as a truncated Fourier sine series) and many other special functions can be described as series involving them.

Based on the fact that many libc implementations include them I think the cost-benefit of exposing them when available is acceptable.