= Maxwell molecule collision operator
{c}
{title2=$Q(f,f)=\frac1{4\pi}\int(f'f_*'-ff_*)\,dv_*\,d\sigma$}
For the normalized isotropic cutoff model, collisions redistribute the direction of relative velocity uniformly while keeping its length. With center $C=(v+v_*)/2$ and relative speed $r=|v-v_*|$, 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.
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