Pk. Anastasovski et al., Operator derivation of the gauge-invariant Proca and Lehnert equations; Elimination of the Lorenz condition, FOUND PHYS, 30(7), 2000, pp. 1123-1129
Using covariant derivatives and the operator definitions of quantum mechani
cs, gauge invariant Proca and Lehnert equations are derived and the Lorenz
condition is eliminated in U(1) invariant electrodynamics. It is shown that
the structure of the gauge invariant Lehnert equation is the same in an O(
3) invariant theory of electrodynamics.