Robuta

https://en.wikibooks.org/wiki/Acoustics/Bessel_Functions_and_the_Kettledrum

Acoustics/Bessel Functions and the Kettledrum - Wikibooks, open books for an open world

bessel functions open books acoustics kettledrum wikibooks

https://www.boost.org/doc/libs/1_60_0/libs/math/doc/html/math_toolkit/bessel/sph_bessel.html

Spherical Bessel Functions of the First and Second Kinds

spherical bessel functions the first second kinds