– Particles are binned once and for all to μ slots according to
their initial pitch-angle, μ(0) ≡ μ0. While at first glance this
procedure seems to be justified by the fact that Eq. (3) is
symmetric in μ(t) and μ0, it has the crucial drawback of conserving
the initial conditions. However, a diffusive process is
defined as a process that has become independent of the initial
condition; therefore, this approach has to be discarded.