1) in your patch for T operator()() part, what's the "n" variable, looking at 1_54_0 version there is no mention to this variable.
There are no changes to operator()(), the n variable appears in the next part of the diff in inline T sph_bessel_j_imp(unsigned n, T x, const Policy& pol)
2) is there a way to get your current version of the bessel.hpp file ?
Attached. Note I had to change the previous fix somewhat as it failed for types with extended exponent ranges (I realised that just as I was dozing off to sleep last night!)
That's true that for x close to zero but positive there are also problems to the use of power series close to 0 is certainly the best thing to do. I am astonished that this not yet used.
Sorry I don't understand: the problem was occurring inside the power series for small x. John.