BESRAT, VKAPPA, SPHERR, CAPPA3: Fortran functions providing statistics for von Mises''s and Fisher''s distributions of directions (the ratio of modified Bessel functions of the first kind). (See G.W. Hill, ACM TOMS 7 (1981) pp. 233-238.).
Modified Bessel functions of imaginary order and positive argument. Computes function values and derivatives. Can also compute exponentially scaled functions. (See A. Gil et al., ACM TOMS 30 (2004) pp. 159-164.).