https://en.wikipedia.org/wiki/Wave_equation#Spherical_waves
Before it states ''where K=w/c'', there is an equation that has the following variables: d,r,w,c,l.
It also has f_lm(r)
What do each of the above mean in this case? I assume that c is the speed of the wave, r is the radial distance of the sphere and that d represents derivative?