Residual Lorenz gauge symmetry
= Residual Lorenz gauge symmetry
{title2=$\Box\lambda=0$}
An Abelian <gauge transformation> $A_a\mapsto A_a+\partial_a\lambda$ changes $\partial\cdot A$ by $\Box\lambda$. Consequently <gauge fixing> by a quadratic function of $\partial\cdot A$ retains transformations satisfying $\Box\lambda=0$, subject to the same <boundary conditions>. Eliminating those residual transformations is part of specifying an inverse with a particular <Green function> prescription.