= Circular infinite quantum well
{title2=$E_{m,j}=\hbar^2j_{|m|,j}^2/(2\mu a^2)$}
A particle confined by a <Dirichlet boundary condition> to a disc of radius $a$ has angular functions $e^{im\phi}$, $m\in\mathbb Z$, and regular radial <Bessel functions of the first kind> $J_{|m|}(kr)$. Their boundary zeros give the displayed energies. The ground state belongs to $m=0$: nonzero angular modes add a nonnegative centrifugal term to the kinetic-energy form. At the origin exclude the singular radial solution, rather than accepting every square-integrable logarithmic function as a physical eigenstate.
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