Three-loop Phi-derivable Approximation in QED
  In this paper we examine Phi-derivable approximations in QED. General theorems tell us that the gauge dependence of the n-loop Phi-derivable approximation shows up at order g^(2n) where g is the coupling constant. We consider the gauge dependence of the two-loop Phi-derivable approximation to the Debye mass and show that it is of order e^4 as expected. We solve the three-loop Phi-derivable approximation in QED by expanding sum-integrals in powers of e^2 and m/T, where m is the Debye mass which satisfies a variational gap equation. The results for the pressure and the Debye mass are accurate to order e^5. 
Jens O. Andersen and Michael Strickland