= Weak Boltzmann collision identity
{title2=$\int Q(f,f)\psi\,dv=\frac12\int Bff_*\Delta\psi\,dv\,dv_*\,dn$}
Here $\Delta\psi=\psi(v')+\psi(v_*')-\psi(v)-\psi(v_*)$ is the pairwise collision change, not a <Laplace operator>. Average over the two particle labels and use collision reversal to obtain the formula. Every <collision invariant> has $\Delta\psi=0$, giving mass, <momentum> and <kinetic energy> conservation. A second symmetrization replaces $ff_*\Delta\psi/2$ by $(f'f_*'-ff_*)(-\Delta\psi)/4$.
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