Asymptotic Expansions of Feynman Amplitudes in a Generic Covariant Gauge
  We show in this paper how to construct Symanzik polynomials and the Schwinger parametric representation of Feynman amplitudes for gauge theories in an unspecified covariant gauge. The complete Mellin representation of such amplitudes is then established in terms of invariants (squared sums of external momenta and squared masses). From the scaling of the invariants by a parameter we extend for the present situation a theorem on asymptotic expansions, previously proven for the case of scalar field theories, valid for both ultraviolet and infrared behaviors of Feynman amplitudes. 
C.A. Linhares, A.P.C. Malbouisson, I. Roditi