An algebraic approach to Borel CSPs

Abstract

We adapt tools from the algebraic approach to constraint satisfaction problems to answer descriptive set theoretic questions about Borel CSPs. We show that if a structure $\mathcal D$ does not have a Taylor polymorphism, then the corresponding Borel CSP is $\mathbf{\Sigma}^1_2$-complete. In particular, by the CSP Dichotomy Theorem, if $\operatorname{CSP}(\mathcal D)$ is $\mathrm{NP}$-complete, then the Borel version, $\operatorname{csp}_B(\mathcal D)$, is $\mathbf{\Sigma}^1_2$-complete (assuming $\mathrm{P}\not=\mathrm{NP}$). We also have partial converses, such as a descriptive analogue of the Hell--Ne\v set\v ril theorem characterizing $\mathbf{\Sigma}^1_2$-complete graph homomorphism problems. We show that the structures where every solvable Borel instance of their CSP has a Borel solution are exactly the width 1 structures. And, we prove a handful of results bounding the projective complexity of certain bounded width structures.