Disproof of the neighborhood conjecture with implications to sat

Abstract

We study a special class of binary trees. Our results have implications on Maker/Breaker games and SAT: We disprove a conjecture of Beck on positional games and construct an unsatisfiable k-CNF formula with few occurrences per variable, thereby improving a previous result by Hoory and Szeider and showing that the bound obtained from the Lovász Local Lemma is tight up to a constant factor. A (k,s)-CNF formula is a boolean formula in conjunctive normal form where every clause contains exactly k literals and every variable occurs in at most s clauses. The (k,s)-SAT problem is the satisfiability problem restricted to (k,s)-CNF formulas. Kratochvíl, Savický and Tuza showed that for every k≥3 there is an integer f(k) such that every (k, f(k))-formula is satisfiable, but (k, f(k)+1)-SAT is already NP-complete (it is not known whether f(k) is computable). Kratochvíl, Savický and Tuza also gave the best known lower bound , which is a consequence of the Lovász Local Lemma. We prove that, in fact, , improving upon the best known upper bound by Hoory and Szeider. Finally we establish a connection between the class of trees we consider and a certain family of positional games. The Maker/Breaker game we study is as follows. Maker and Breaker take turns in choosing vertices from a given n-uniform hypergraph , with Maker going first. Maker's goal is to completely occupy a hyperedge and Breaker tries to prevent this. Beck conjectures that if the maximum neighborhood size of is smaller than 2 n-1-1 then Breaker has a winning strategy. We disprove this conjecture by establishing an n-uniform hypergraph with maximum neighborhood size 3 •2 n-3 where Maker has a winning strategy. Moreover, we show how to construct an n-uniform hypergraph with maximum degree where Maker has a winning strategy. In addition we show that each n-uniform hypergraph with maximum degree at most has a proper halving 2-coloring, which solves another open problem posed by Beck related to the Neighborhood Conjecture. © 2009 Springer Berlin Heidelberg.