Going after the k-SAT Threshold

TL;DR

This paper introduces a new asymmetric second moment method to precisely determine the satisfiability threshold for random k-SAT problems, narrowing the gap between known bounds and matching predictions from physics-based methods.

Contribution

The authors develop an innovative asymmetric second moment technique that accurately computes the k-SAT threshold, improving upon previous bounds and aligning with non-rigorous physics predictions.

Findings

Achieves a threshold estimate within approximately 0.19 of the true value.

Develops the first rigorous asymmetric second moment method for random CSPs.

Matches the replica symmetric bound from the cavity method.

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 non-rigorous technique called the "cavity method" for the study of random CSPs (M\'ezard, Parisi, Zecchina: Science 2002). Our result matches the best bound that can be obtained from the so-called "replica symmetric" version of the cavity method, and indeed our proof directly harnesses parts of the physics calculations.