XI-E's class and admission, copied

Since $\mathcal{P}$ is an invertible constant matrix, $-\square \mathcal{P}\gamma =0$ is $\square \gamma =0$ componentwise: every $\gamma =ϵe^{ik\cdot y}$ with constant symmetric $ϵ$ and null $k$ is a free metric configuration, for every polarization $ϵ$.