= Gauge-fixed Maxwell kinetic operator
{title2=$K^{\mu\nu}=-k^2\eta^{\mu\nu}+(1-\xi^{-1})k^\mu k^\nu$}
The Fourier-space quadratic Maxwell kernel after adding $-(\partial_\mu A^\mu)^2/(2\xi)$. For nonzero $\xi$ and non-null <four-momentum>, its <transverse and longitudinal momentum projectors> give $K=-k^2(P_T+\xi^{-1}P_L)$ and its inverse is $-(P_T+\xi P_L)/k^2$. The <inversion of the gauge-fixed Maxwell kinetic operator> supplies the <photon propagator> with its vacuum boundary prescription.
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