Constraint and Satisfiability Reasoning for Graph Coloring

Abstract

Graph coloring is an important problem in combinatorial optimization and a major component of numerous allocation and scheduling problems. In this paper we introduce a hybrid CP/SAT approach to graph coloring based on the addition-contraction recurrence of Zykov. Decisions correspond to either adding an edge between two non-adjacent vertices or contracting these two vertices, hence enforcing inequality or equality, respectively. This scheme yields a symmetry-free tree and makes learnt clauses stronger by not committing to a particular color. We introduce a new lower bound for this problem based on Mycielskian graphs; a method to produce a clausal explanation of this bound for use in a CDCL algorithm; a branching heuristic emulating Br´elaz’ heuristic on the Zykov tree; and dedicated pruning techniques relying on marginal costs with respect to the bound and on reasoning about transitivity when unit propagating learnt clauses. The combination of these techniques in both a branch-and-bound and in a bottom-up search outperforms other SAT-based approaches and Dsatur on standard benchmarks both for finding upper bounds and for proving lower bounds.

Total Coloring and Total Matching: Polyhedra and Facets

“…constraint satisfiability [13], memetic algorithms [24]), and other interesting types of coloring problems (e.g., equitable coloring [22,4], graph multicoloring [11], sum coloring [5], selective graph coloring [6]), in this work, we focus on a polyhedral approach to the Total Coloring Problem.…”