While the calculation of the mean free path values is
straightforward, this is somewhat different for the Fokker-Planckcoefficient(s). Especially Dμμ has to be a function of time but, at
the same time, depends on the μ values. It has to be stressed that
pitch-angle scattering is unique in that, unlike for normal spatial
diffusion, the coefficient depends on the variable from which the
mean square displacement is obtained.