Constraint satisfaction problems for reducts of homogeneous graphs

Bodirsky, M. and Martin, B. and Pinsker, M. and Pongrácz, András (2019) Constraint satisfaction problems for reducts of homogeneous graphs. SIAM JOURNAL ON COMPUTING, 48 (4). pp. 1224-1264. ISSN 0097-5397

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Abstract

For n >= 3, let (H-n, E) denote the nth Henson graph, i.e., the unique countable homogeneous graph with exactly those finite graphs as induced subgraphs that do not embed the complete graph on n vertices. We show that for all structures Gamma with domain H-n whose relations are first-order definable in (H-n, E) the constraint satisfaction problem for F either is in P or is NP-complete. We moreover show a similar complexity dichotomy for all structures whose relations are first-order definable in a homogeneous graph whose reflexive closure is an equivalence relation. Together with earlier results, in particular for the random graph, this completes the complexity classification of constraint satisfaction problems of structures first-order definable in countably infinite homogeneous graphs: all such problems are either in P or NP-complete.

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