  We study the O(N) vector model and the U(N) Gross-Neveu model with fixed
total fermion number, in three dimensions. Using non-trivial polylogarithmic
identities, we calculate the large-N renormalized free-energy density of these
models, at their conformal points in a ``slab'' geometry with one finite
dimension of length L. We comment on the possible implications of our results.
