Loop corrections for hard spheres in Hamming space

TL;DR

This paper develops a theoretical framework using Belief Propagation to analyze the entropy and packing density of hard spheres in Hamming space, supporting conjectures about optimal packing limits.

Contribution

It introduces an extended BP-based method incorporating loopy interactions to accurately estimate entropy and packing density in Hamming space.

Findings

Supports Gilbert-Varshamov lower bound for maximum packing density

Validates loopy BP equations as accurate approximation methods

Provides a new analytical approach for high-dimensional sphere packings

Abstract

We begin with an exact expression for the entropy of a system of hard spheres within the Hamming space. This entropy relies on probability marginals, which are determined by an extended set of Belief Propagation (BP) equations. The BP probability marginals are functions of auxiliary variables which are introduced to model the effects of loopy interactions on a tree-structured interaction graph. We explore various reasonable and approximate probability distributions, ensuring they align with the exact solutions of the BP equations. Our approach is based on an ansatz of (in)homogeneous cavity marginals respecting the permutation symmetry of the problem. Through thorough analysis, we aim to minimize errors in the BP equations. Our findings support the conjecture that the maximum packing density asymptotically conforms to the lower bound proposed by Gilbert and Varshamov, further validated by the solution of the loopy BP equations.