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Effectiveness of Structural Restrictions for Hybrid CSPs

Constraint Satisfaction Problem (CSP) is a fundamental algorithmic problem that appears in many areas of Computer Science. It can be equivalently stated as computing a homomorphism $\mbox{$\bR \rightarrow \bGamma$}$ between two relational structures, e.g.\ between two directed graphs. Analyzing its complexity has been a prominent research direction, especially for {\em fixed template CSPs} in which the right side $\bGamma$ is fixed and the left side $\bR$ is unconstrained. Far fewer results are known for the {\em hybrid} setting that restricts both sides simultaneously. It assumes that $\bR$ belongs to a certain class of relational structures (called a {\em structural restriction} in this paper). We study which structural restrictions are {\em effective}, i.e.\ there exists a fixed template $\bGamma$ (from a certain class of languages) for which the problem is tractable when $\bR$ is restricted, and NP-hard otherwise. We provide a characterization for structural restrictions that are {\em closed under inverse homomorphisms}. The criterion is based on the {\em chromatic number} of a relational structure defined in this paper; it generalizes the standard chromatic number of a graph. As our main tool, we use the algebraic machinery developed for fixed template CSPs. To apply it to our case, we introduce a new construction called a "lifted language." We also give a characterization for structural restrictions corresponding to minor-closed families of graphs, extend results to certain Valued CSPs (namely conservative valued languages), and state implications for CSPs with ordered variables, (valued) CSPs on structures with large girth, and for the maximum weight independent set problem on some restricted families of graphs including graphs with large girth.