= Moving-surface retarded Jacobian
{title2=$|1-M_r|^{-1}$}
For a source surface parameterized by $y(p,q,\tau)$, the <retarded time> equation is $t=\tau+|x-y(p,q,\tau)|/c_0$. Integrating a <Dirac delta distribution> enforcing that equation produces the reciprocal <derivative> $|1-M_r|^{-1}$ at each simple root. For subsonic motion there is a unique retarded root. Multiple roots require summation; a zero <derivative> is outside the simple-root rule. This factor is the moving-source counterpart of a delta change-of-variable Jacobian.
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