The Dirac equation for an electron in an external electromagnetic field can
be regarded as a singular set of linear equations for the vector potential
. Radford's method of algebraically solving for the vector potential is rev
iewed, with attention to the additional constraints arising from non-maxima
lity of the rank. The extension of the method to general spacetimes is illu
strated by examples in diverse dimensions with both c- and a-number wavefun
ctions.