  We derive a differential equation for the one-particle-irreducible vertex
functions of interacting fermions as a function of the temperature. Formally,
these equations correspond to a Wilsonian renormalization group scheme which
uses the temperature as an explicit scale parameter. Our novel method allows us
to analyze the competition between superconducting and various magnetic Fermi
surface instabilities in the one-loop approximation. In particular this
includes ferromagnetic fluctuations, which are difficult to treat on an equal
footing in conventional Wilsonian momentum space techniques. Applying the
scheme to the two-dimensional t-t' Hubbard model we investigate the RG flow of
the interactions at the van Hove filling with varying next-nearest neighbor
hopping t'. Starting at t'=0 we describe the evolution of the flow to strong
coupling from an antiferromagnetic nesting regime over a d-wave regime at
moderate t' to a ferromagnetic region at larger absolute values of t'. Upon
increasing the particle density in the latter regime the ferromagnetic
tendencies are cut off and the leading instability occurs in the triplet
superconducting pairing channel.
