From algorithms to connectivity and back: finding a giant component in random k-SAT

TL;DR

This paper investigates the structure of solution spaces in sparse random and bounded degree k-CNFs, establishing the existence of a giant connected component of solutions and proposing an efficient sampling algorithm.

Contribution

It introduces new probabilistic results on the connectivity of solutions in random k-CNFs and develops a polynomial-time sampling algorithm based on spectral independence.

Findings

Existence of a giant component in solution space for certain densities

Development of a polynomial-time solution sampling algorithm

Fast mixing properties of the proposed Markov chain

Abstract

We take an algorithmic approach to studying the solution space geometry of relatively sparse random and bounded degree $k$-CNFs for large $k$. In the course of doing so, we establish that with high probability, a random $k$-CNF $\Phi$ with $n$ variables and clause density $\alpha = m/n \lesssim 2^{k/6}$ has a giant component of solutions that are connected in a graph where solutions are adjacent if they have Hamming distance $O_k(\log n)$ and that a similar result holds for bounded degree $k$-CNFs at similar densities. We are also able to deduce looseness results for random and bounded degree $k$-CNFs in a similar regime. Although our main motivation was understanding the geometry of the solution space, our methods have algorithmic implications. Towards that end, we construct an idealized block dynamics that samples solutions from a random $k$-CNF $\Phi$ with density $\alpha = m/n \lesssim 2^{k/52}$. We show this Markov chain can with high probability be implemented in polynomial time and by leveraging spectral independence, we also observe that it mixes relatively fast, giving a polynomial time algorithm to with high probability sample a uniformly random solution to a random $k$-CNF. Our work suggests that the natural route to pinning down when a giant component exists is to develop sharper algorithms for sampling solutions in random $k$-CNFs.