Near-Optimal Cayley Expanders for Abelian Groups

TL;DR

This paper presents a deterministic algorithm for constructing near-optimal expanding generating sets for finite abelian groups, improving efficiency and resolving a longstanding conjecture, with applications in derandomization and complexity theory.

Contribution

It introduces a new deterministic construction of Cayley expanders for abelian groups with optimal dependence on dimension, extending bias amplification techniques.

Findings

Constructs near-optimal Cayley expanders efficiently

Resolves a conjecture on dependence on dimension

Enables applications in derandomization and complexity

Abstract

We give an efficient deterministic algorithm that outputs an expanding generating set for any finite abelian group. The size of the generating set is close to the randomized construction of Alon and Roichman (1994), improving upon various deterministic constructions in both the dependence on the dimension and the spectral gap. By obtaining optimal dependence on the dimension we resolve a conjecture of Azar, Motwani, and Naor (1998) in the affirmative. Our technique is an extension of the bias amplification technique of Ta-Shma (2017), who used random walks on expanders to obtain expanding generating sets over the additive group of n-bit strings. As a consequence, we obtain (i) randomness-efficient constructions of almost k-wise independent variables, (ii) a faster deterministic algorithm for the Remote Point Problem, (iii) randomness-efficient low-degree tests, and (iv) randomness-efficient verification of matrix multiplication.