Andrei A. Bulatov
April 2002, 47pp.
The Constraint Satisfaction Problem (CSP) provides a common framework for many combinatorial problems. The general CSP is known to be NP-complete; however, certain restrictions on a possible form of constraints may affect the complexity, and lead to tractable problem classes. There is, therefore, a fundamental research direction, aiming to separate those subclasses of the CSP which are tractable, and those which remain NP-complete.
Schaefer gave an exhaustive solution of this problem for the CSP on a 2-element domain. In this paper we generalise this result to a classification of the complexity of CSPs on a 3-element domain. The main result states that every subclass of the CSP is either tractable or NP-complete. A polynomial time algorithm is also exhibited which, for a given set of allowed constraints, outputs if this set gives rise to a tractable problem class.