id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
4812	Bug in boost::math::special_functions::sign showing up in special_functions::bessel_jy()	peter zwart <PHZwart@…>	John Maddock	"Hi,

When computing scattering curves, I encountered a show-stopping bug that I tracked back to the boost::math::special_functions::sign function .

In math::special_functions::bessel_jy a small number is generated via 

sqrt(tools::min_value<T>()) 

which subsequently ends up in this statement:  sign(current)

I see this happening:

variable current: -1.24911e-2468
variable sign(current): 1

which causes problems for math::special_functions::sph_bessel(1,x) for x between 7.845 and 8 or so on my machines.

The fix that worked for me is the following

          T sign_current;
          sign_current = std::fabs(current);
          if (current*2.0 < sign_current){
            sign_current = -1.0;
          } else {
            sign_current = 1.0;
          }

          Ju = sign_current * sqrt(W / (q + gamma * (p - t)));
          // Ju = sign(current) * sqrt(W / (q + gamma * (p - t)));


For now, I'll run with this local fix but it seems that on some platforms the current sign function causes problems when computing spherical Bessel functions of any order above 0.







"	Bugs	closed	To Be Determined	math	Boost 1.45.0	Showstopper	worksforme	sign sph_bessel special_function