 Maxwell molecule collision operator

For the normalized isotropic cutoff model, collisions redistribute the direction of relative velocity uniformly while keeping its length. With center $C=(v+v_{\ast })/2$ and relative speed $r=|v-v_{\ast }|$, outgoing velocities are $C\pm r\sigma /2$. Momentum and kinetic energy are preserved, and the collision rate has no relative-speed factor. The displayed operator uses unit total angular rate. General Maxwell molecule kernels can have nonconstant angular dependence.