zeros of Bessel J function about specialfunctions.jl

[jtravs]

I'd find this really useful.

[jlapeyre]

I added support for Bessel functions of the second kind bessely_zero to FunctionZeros.jl. In testing it, I discovered an error in bessely in mpath.

I mentioned it might be kinda slow because I only use root finding, use a slow root finder, don't supply derivatives, etc. But, its still 100x faster than mpmath.

In [4]: import mpmath

In [5]: %timeit mpmath.besseljzero(1.0,1)
1000 loops, best of 3: 484 µs per loop

julia> using FunctionZeros; using BenchmarkTools;

julia> @btime besselj_zero(1.0,1)
  3.018 μs (48 allocations: 2.16 KiB)
3.8317059702075125