= Kuzmin reflection method
{c}
{title2=$\Sigma(R)=-\partial_z\psi(R,0^+)/(2\pi G)$}
Reflect a vacuum <gravitational potential> in the upper half-space evenly across $z=0$. For the convention in which <gravitational acceleration> is $\nabla\psi$, the derivative jump is $2\partial_z\psi(R,0^+)=-4\pi G\Sigma(R)$. Thus a point mass $m$ below the plane produces a <Kuzmin disk>, and superposing point masses with weight $\mu(b)\,db$ produces $\Sigma(R)=(2\pi)^{-1}\int_0^\infty b\mu(b)(R^2+b^2)^{-3/2}\,db$. Positive weights give a nonnegative <surface density> wherever the integral converges.
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