Going after the kSAT Threshold
Abstract
Random $k$SAT is the single most intensely studied example of a random constraint satisfaction problem. But despite substantial progress over the past decade, the threshold for the existence of satisfying assignments is not known precisely for any $k\geq3$. The best current results, based on the second moment method, yield upper and lower bounds that differ by an additive $k\cdot \frac{\ln2}2$, a term that is unbounded in $k$ (Achlioptas, Peres: STOC 2003). The basic reason for this gap is the inherent asymmetry of the Boolean value `true' and `false' in contrast to the perfect symmetry, e.g., among the various colors in a graph coloring problem. Here we develop a new asymmetric second moment method that allows us to tackle this issue head on for the first time in the theory of random CSPs. This technique enables us to compute the $k$SAT threshold up to an additive $\ln2\frac12+O(1/k)\approx 0.19$. Independently of the rigorous work, physicists have developed a sophisticated but nonrigorous technique called the "cavity method" for the study of random CSPs (Mézard, Parisi, Zecchina: Science 2002). Our result matches the best bound that can be obtained from the socalled "replica symmetric" version of the cavity method, and indeed our proof directly harnesses parts of the physics calculations.
 Publication:

arXiv eprints
 Pub Date:
 December 2012
 arXiv:
 arXiv:1212.1682
 Bibcode:
 2012arXiv1212.1682C
 Keywords:

 Mathematics  Combinatorics;
 Computer Science  Discrete Mathematics
 EPrint:
 Proc. 45 th STOC (2013) 705  714