  The basic AB problem is to determine how an unshielded tube of magnetic flux
$\Phi$ affects arbitrarily long-wavelength charged particles impinging on it.
For spin-1 at almost all $\Phi$ the particles do not penetrate the tube, so the
interaction essentially is periodic in $\Phi$ (AB effect). Below-threshold
bound states move freely only along the tube axis, and consequent induced
vacuum currents supplement rather than screen $\Phi$. For a pure magnetic
interaction the tube must be broader than the particle Compton wavelength,
i.e., only the nonrelativistic spin-1 AB problem exists.
