  The naive perturbation expansion for many-fermion systems is infrared
divergent. One can remove these divergences by introducing counterterms. To do
this without changing the model, one has to solve an inversion equation. We
call this procedure Fermi surface renormalization (FSR). Whether or not FSR is
possible depends on the regularity properties of the fermion self--energy. When
the Fermi surface is nonspherical, this regularity problem is rather
nontrivial. Using improved power counting at all orders in perturbation theory,
we have shown sufficient differentiability to solve the FSR equation for a
class of models with a non-nested, non-spherical Fermi surface. I will first
motivate the problem and give a definition of FSR, and then describe the
combination of geometric and graphical facts that lead to the improved power
counting bounds. These bounds also apply to the four--point function. They
imply that only ladder diagrams can give singular contributions to the
four--point function.

Condensed Matter