Normalized velocity-reset projection ($Pg=\rho (g)M,P^2=P$)

For a nonnegative normalized velocity density $M$, $Pg(x,v)=M(v)\int g(x,w)dw$ is a positive norm-one projection on phase-space $L^1$. Normalization gives $P^2=P$. Its range consists of fields $a(x)M(v)$. This Banach-space projection need not be an orthogonal projection in unweighted $L^2$; no such assertion is needed for the $L^1$ collision estimates.