= Cube flux from a corner charge
{title2=$\Phi_{\rm opposite}=q/(24\epsilon_0)$}
For a <point charge> exactly at a cube corner, each of the three opposite faces has outward <electric flux> $q/(24\epsilon_0)$. Reflecting the cube in its three incident faces makes a larger cube centred on the charge and proves the result by <symmetry>. The three incident faces have zero normal <electric field> away from their singular corner. Their improper integrals vanish; the total is $q/(8\epsilon_0)$. This boundary convention differs from moving a charge to the corner from the interior after computing a closed-surface integral.
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