  The instabilities induced on a two-dimensional system of correlated electrons
by the anisotropies of its Fermi line are analyzed on general grounds. Simple
scaling arguments allow to predict the opening of a superconducting gap with a
well-defined symmetry prescribed by the geometry of the Fermi line. The same
arguments predict a critical dimension of 3/2 for the transition of the
two-dimensional system to non-Fermi liquid behavior. The methods are applied to
the t-t' Hubbard model in a wide range of dopings.
