Department of Computer Science | Institute of Theoretical Computer Science | CADMO

Theory of Combinatorial Algorithms

Prof. Emo Welzl and Prof. Bernd Gärtner

Mittagsseminar (in cooperation with M. Ghaffari, A. Steger and B. Sudakov)

Mittagsseminar Talk Information

Date and Time: Thursday, September 18, 2008, 12:15 pm

Duration: This information is not available in the database

Location: CAB G51

Speaker: Robin Moser

Derandomizing the Lovász Local Lemma more Effectively

The famous Lovász Local Lemma is a powerful tool to non-constructively prove the existence of combinatorial objects meeting a prescribed collection of criteria. Kratochvíl et al. applied this technique to prove that a k-CNF in which each variable appears at most 2^k/(ek) times is always satisfiable. In a breakthrough paper, Beck found that if we lower the occurrences to O(2^(k/48)/k), then a deterministic polynomial-time algorithm can find a satisfying assignment to such an instance. Alon randomized the algorithm and required O(2^(k/8)/k) occurrences. In 2006, we exhibited a refinement of his method which copes with O(2^(k/6)/k) of them. The hitherto best known randomized algorithm is due to Srinivasan and is capable of solving O(2^(k/4)/k) occurrence instances. Answering two questions asked by Srinivasan, we shall now present an approach that tolerates O(2^(k/2)/k) occurrences per variable and which can most easily be derandomized. The new algorithm bases on an alternative type of witness tree structure and drops a number of limiting aspects common to all previous methods.

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