Factorization of covariant Feynman graphs for the effective action

We prove a neat factorization property of Feynman graphs in covariant perturbation theory. The contribution of the graph to the effective action is written as a product of a massless scalar momentum integral that only depends on the basic graph topology, and a background-field dependent piece that contains all the information of spin, gauge representations, masses etc. We give a closed expression for the momentum integral in terms of four graph polynomials whose properties we derive in some detail. Our results can also be useful for standard (non-covariant) perturbation theory.