Typical sofic entropy and local limits for free group shift systems

Abstract

We show that for any invariant measure $\mu$ on a free group shift system, there are two numbers $h^\flat \leq h^\sharp$ which in some sense are the typical upper and lower sofic entropy values. We also give a condition under which $h^\flat = h^\sharp = \mathrm{f}(\mu)$, where $\mathrm{f}$ is the annealed entropy (also called the f invariant). This can be used to compute typical local limits of finitary Gibbs states over sequences of random regular graphs. As examples, we work out typical local limits of the Ising and Potts models. We also show that, for Markov chains, the Kesten--Stigum second-eigenvalue reconstruction criterion actually implies there are no good models over a typical sofic approximation (i.e. $h^\sharp = -\infty$). In particular, we have an exact value for the typical entropy $h^\flat = h^\sharp$ of the free-boundary Ising state: it is equal to the annealed entropy $\mathrm{f}$ for interaction strengths up to the reconstruction threshold, after which it drops abruptly to $-\infty$.