Below is the ascii version of the abstract for 14-74. The html version should be ready soon.

P. Butera, P. Federbush, M. Pernici
Positivity of the virial coefficients in lattice dimer models
(34K, LaTeX)

ABSTRACT.   Using a simple relation between the virial expansion coefficients of 
 the pressure and the entropy expansion coefficients in the case of the 
 monomer-dimer model on infinite regular lattices, we have shown that, on 
 hypercubic lattices of any dimension, the virial coefficients are 
 positive through the 20th order. We have also observed that all 
 virial coefficients so far known for this system are positive also 
 on various infinite regular lattices of a different structure. We are thus 
 led to conjecture that all of them are always positive. 
 These considerations are generalized to the study of related bounds 
 on finite graphs bearing some similarity to infinite regular lattice 
 models, namely regular biconnected graphs and finite grids. 
 The validity of the bounds Delta^k (ln(i! N(i))) < 0 for 
 k > 1, where N(i) is the number of configurations of i 
 dimers on the graph and Delta is the finite difference operator, 
 is shown to correspond to the positivity of the virial coefficients. 
 An exhaustive survey of some classes of regular biconnected graphs 
 with a not too large number v of vertices shows that there are 
 only few violations of these bounds. We conjecture that the 
 frequency of the violations vanishes as v goes to infinity. 
 These bounds are valid also for square and triangular N x N 
 grids with N < 20 or N < 19 respectively, and with open 
 boundary conditions, giving some support to the 
 conjecture on the positivity of the virial coefficients.