A conformally invariant Maxwell gauge (Q5951594)

A gauge condition for Maxwell potentials \(\Phi_a\) on a spacetime with metric \(g_{ab}\colon \nabla_a[\nabla^a\nabla^b- 2R^{ab}+\frac 23Rg^{ab}]\Phi_a=0\) is derived that is conformally invariant if \(\Phi_a\) is source-free: \(\nabla^a\nabla_a\Phi_b-\nabla^a\nabla_b\Phi_a=0\).