Unit flow
= Unit flow
{title2=$\operatorname{div}\vartheta=\mathbf1_a-\mathbf1_b$}
A unit flow from $a$ to $b$ is an antisymmetric <flow> with net outward <strength of a flow> one at $a$, net inward strength one at $b$, and zero <divergence of a flow> elsewhere. For a unit flow to infinity, only the finite source has nonzero divergence.