Approximating partition functions of bounded-degree Boolean counting Constraint Satisfaction Problems

We study the complexity of approximate counting Constraint Satisfaction Problems (#CSPs) in a bounded degree setting. Specifically, given a Boolean constraint language $\Gamma$ and a degree bound $\Delta$, we study the complexity of #CSP$_\Delta(\Gamma)$, which is the problem of counting satisfying assignments to CSP instances with constraints from $\Gamma$ and whose variables can appear at most $\Delta$ times. Our main result shows that: (i) if every function in $\Gamma$ is affine, then #CSP$_\Delta(\Gamma)$ is in FP for all $\Delta$, (ii) otherwise, if every function in $\Gamma$ is in a class called IM$_2$, then for all sufficiently large $\Delta$, #CSP$_\Delta(\Gamma)$ is equivalent under approximation-preserving (AP) reductions to the counting problem #BIS (the problem of counting independent sets in bipartite graphs) (iii) otherwise, for all sufficiently large $\Delta$, it is NP-hard to approximate the number of satisfying assignments of an instance of #CSP$_\Delta(\Gamma)$, even within an exponential factor. Our result extends previous results, which apply only in the so-called "conservative" case.