CSP dichotomy for special triads

Barto, Libor; Kozik, Marcin; Maróti, Miklós; Niven, Todd

doi:10.1090/S0002-9939-09-09883-9

CSP dichotomy for special triads
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by Libor Barto, Marcin Kozik, Miklós Maróti and Todd Niven PDF

Proc. Amer. Math. Soc. 137 (2009), 2921-2934 Request permission

Abstract:

For a fixed digraph $\mathbb {G}$, the Constraint Satisfaction Problem with the template $\mathbb {G}$, or $\mathrm {CSP}(\mathbb {G})$ for short, is the problem of deciding whether a given input digraph $\mathbb {H}$ admits a homomorphism to $\mathbb {G}$. The dichotomy conjecture of Feder and Vardi states that $\mathrm {CSP}(\mathbb {G})$, for any choice of $\mathbb G$, is solvable in polynomial time or NP-complete. This paper confirms the conjecture for a class of oriented trees called special triads. As a corollary we get the smallest known example of an oriented tree (with $33$ vertices) defining an NP-complete $\mathrm {CSP}(\mathbb {G})$.

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