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Transfer matrix computation of critical polynomials for two-dimensional Potts models

In our previous work we have shown that critical manifolds of the q-state Potts model can be studied by means of a graph polynomial P_B(q,v), henceforth referred to as the critical polynomial. This polynomial may be defined on any periodic two-dimensional lattice. It depends on a finite subgraph B, called the basis, and the manner in which B is tiled to construct the lattice. The real roots v = e^K - 1 of P_B(q,v) either give the exact critical points for the lattice, or provide approximations that, in principle, can be made arbitrarily accurate by increasing the size of B in an appropriate way. In earlier work, P_B(q,v) was defined by a contraction-deletion identity, similar to that satisfied by the Tutte polynomial. Here, we give a probabilistic definition of P_B(q,v), which facilitates its computation, using the transfer matrix, on much larger B than was previously possible. We present results for the critical polynomial on the (4,8^2), kagome, and (3,12^2) lattices for bases of up to respectively 96, 162, and 243 edges, compared to the limit of 36 edges with contraction-deletion. We discuss in detail the role of the symmetries and the embedding of B. The critical temperatures v_c obtained for ferromagnetic (v>0) Potts models are at least as precise as the best available results from Monte Carlo simulations or series expansions. For instance, with q=3 we obtain v_c(4,8^2) = 3.742 489(4), v_c(kagome) = 1.876 459 7(2), and v_c(3,12^2) = 5.033 078 49(4), the precision being comparable or superior to the best simulation results. More generally, we trace the critical manifolds in the real (q,v) plane and discuss the intricate structure of the phase diagram in the antiferromagnetic (v<0) region.