Faster algorithm for Unique $(k,2)$-CSP

Abstract

In a $(k,2)$-Constraint Satisfaction Problem we are given a set of arbitrary constraints on pairs of $k$-ary variables, and are asked to find an assignment of values to these variables such that all constraints are satisfied. The $(k,2)$-CSP problem generalizes problems like $k$-coloring and $k$-list-coloring. In the Unique $(k,2)$-CSP problem, we add the assumption that the input set of constraints has at most one satisfying assignment. Beigel and Eppstein gave an algorithm for $(k,2)$-CSP running in time $O\left(\left(0.4518k\right)^n\right)$ for $k>3$ and $O\left(1.356^n\right)$ for $k=3$, where $n$ is the number of variables. Feder and Motwani improved upon the Beigel-Eppstein algorithm for $k\geq 11$. Hertli, Hurbain, Millius, Moser, Scheder and Szedl{\'a}k improved these bounds for Unique $(k,2)$-CSP for every $k\geq 5$. We improve the result of Hertli et al. and obtain better bounds for Unique~$(k,2)$-CSP for~$k\geq 5$. In particular, we improve the running time of Unique~$(5,2)$-CSP from~$O\left(2.254^n\right)$ to~$O\left(2.232^n\right)$ and Unique~$(6,2)$-CSP from~$O\left(2.652^n\right)$ to~$O\left(2.641^n\right)$.