= Orbit equation for combined inverse-square and inverse-cube attraction
{title2=$u''+(1-B/l^2)u=A/l^2$}
For radial acceleration $-A/r^2-B/r^3$, conserved <specific angular momentum> $l$ and $u=1/r$ reduce the <Binet equation> to the displayed linear equation. If $l^2>B$, its angular oscillation frequency is $\sqrt{1-B/l^2}$; attraction proportional to $r^{-3}$ changes the apsidal angle. A zero of $u$ ends the physical positive-radius branch and corresponds to escape, not passage to a negative radius. A spatial self-intersection need not be a periodic return because <velocity> may differ.
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