Inversion of the gauge-fixed Maxwell kinetic operator
= Inversion of the gauge-fixed Maxwell kinetic operator
{title2=$K^{\mu\nu}D^F_{\nu\rho}=i\delta^\mu{}_{\rho}$}
<Gauge fixing> removes the noninvertible longitudinal direction of the Maxwell quadratic action. In a gauge with parameter $\xi$, the Fourier kernel is $-k^2\eta^{\mu\nu}+(1-\xi^{-1})k^\mu k^\nu$. Multiplying its inverse by $i$ gives the <photon propagator>, with the <Feynman i-epsilon prescription>. At $\xi=1$ the answer is $-i\eta_{\mu\nu}/(k^2+i0)$.