= Positive-sign covariant gauge-fixing inverse
{title2=$G_{ab}=-\eta_{ab}/p^2+(1+\xi)p_ap_b/(p^2)^2$}
For fixed $\xi\ne0$, the Lorentzian quadratic density $-F^2/4+(\partial\cdot A)^2/(2\xi)$ has momentum kernel $K^{ab}=-p^2\eta^{ab}+(1+1/\xi)p^ap^b$. At $p^2\ne0$ its inverse, defined by $K^{ac}G_{cb}=\delta^a_b$, is $G_{ab}=-\eta_{ab}/p^2+(1+\xi)p_ap_b/(p^2)^2$. A vacuum <photon propagator> is $iG$ with the appropriate <Feynman i-epsilon prescription>. The usual parameter in the negative-sign <covariant gauge> term is $\alpha=-\xi$, so <Feynman gauge> is $\xi=-1$.
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