  In the framework of a multiorbital Hubbard model description of
superconductivity, a matrix formulation of the superconducting pairing
interaction that has been widely used is designed to treat spin, charge and
orbital fluctuations within a random phase approximation (RPA). In terms of
Feynman diagrams, this takes into account particle-hole ladder and bubble
contributions as expected. It turns out, however, that this matrix formulation
also generates additional terms which have the diagrammatic structure of vertex
corrections. Here we examine these terms and discuss the relationship between
the matrix-RPA superconducting pairing interaction and the Feynman diagrams
that it sums.
