Sampling colorings almost uniformly in sparse random graphs

The problem of sampling proper $q$-colorings from uniform distribution has been extensively studied. Most of existing samplers require $q\ge αΔ+β$ for some constants $α$ and $β$, where $Δ$ is the maximum degree of the graph. The problem becomes more challenging when the underlying graph has unbounded degree since even the decision of $q$-colorability becomes nontrivial in this situation. The Erdős-Rényi random graph $\mathcal{G}(n,d/n)$ is a typical class of such graphs and has received a lot of recent attention. In this case, the performance of a sampler is usually measured by the relation between $q$ and the average degree $d$. We are interested in the fully polynomial-time almost uniform sampler (FPAUS) and the state-of-the-art with such sampler for proper $q$-coloring on $\mathcal{G}(n,d/n)$ requires that $q\ge 5.5d$. In this paper, we design an FPAUS for proper $q$-colorings on $\mathcal{G}(n,d/n)$ by requiring that $q\ge 3d+O(1)$, which improves the best bound for the problem so far. Our sampler is based on the spatial mixing property of $q$-coloring on random graphs. The core of the sampler is a deterministic algorithm to estimate the marginal probability on blocks, which is computed by a novel block version of recursion for $q$-coloring on unbounded degree graphs.