The problem:
$$\mathcal{L} = F^{\mu \nu} F_{\mu \nu} + m^2 /2 \ A_{\mu} A^{\mu} $$
with: $$ F_{\mu \nu} = \partial_{\nu}A_{\mu} - \partial_{\mu}A_{\nu} $$
1. Show that this lagrangian density is not gauge invariance
2.Derive the equations of motion, why is the Lorentzcondition still...