Conserving approximations are applied to the attractive Holstein and Hubbard models (on an infinite-dimensional hypercubic lattice). All effects of nonconstant density of states and vertex corrections are taken into account in the weak-coupling regime. Infinite summation of certain classes of diagrams turns out to be a quantitatively less accurate approximation than truncation of the conserving approximations to a finite order, but the infinite summation approximations do show the correct qualitative behavior of generating a peak in the transition temperature as the interaction strength increases.

• Received 27 January 1994

DOI:https://doi.org/10.1103/PhysRevB.50.403