A polynomial algorithm for 3-Compatible Coloring and the Stubborn List Partition problem (the Stubborn problem is Stubborn no more)

摘要

One of the driving problems in the CSP area is the dichotomy conjecture, formulated in 1993 by Feder and Vardi [Monotone monadic SNP and constraint satisfaction, in Proceedings of the 25th Annual Symposium on Theory of Computing, ACM, New York, 1993, pp. 612-622], stating that for any fixed relational structure Γ the constraint satisfaction problem CSP(γ) is either NP-complete or polynomial time solvable. A large amount of research has gone into checking various specific cases of this conjecture. One such variant which attracted a lot of attention in recent years is the List Matrix Partition problem. In 2004 Cameron et al. [SIAM J. Discrete Math., 21(2007), pp. 900-929] classified almost all List Matrix Partition variants for matrices of size at most four. The only case which resisted the classification became known as the Stubborn problem. In this paper we show a result which enables us to finish the classification-thus solving a problem which resisted attacks for a few years. Our approach is based on a combinatorial problem known to be at least as hard as the Stubborn problem-the 3-Compatible Coloring problem. In this problem we are given a complete graph with each edge assigned one of three possible colors and we want to assign one of those three colors to each vertex in such a way that no edge has the same color as both of its endpoints. The tractability of the 3-Compatible Coloring problem has been open for several years and the best known algorithm prior to this paper is due to Feder et al. [Two algorithms for general list matrix partitions, in Proceedings of the 16th Annual ACM-SIAM Symposium on Discrete Algorithms, ACM, New York, SIAM, Philadelphia, 2005, pp. 870-876]-a quasipolynomial algorithm with a n ~(O(log n/log log n)) time complexity. In this paper we present a polynomial-time algorithm for the 3-Compatible Coloring problem and consequently we prove a dichotomy for the k-Compatible Coloring problem.