A Near-Cubic Lower Bound for 3-Query Locally Decodable Codes from Semirandom CSP Refutation

TL;DR

This paper establishes a near-cubic lower bound on the length of 3-query locally decodable codes, advancing understanding of their limitations and connecting them to CSP refutation techniques.

Contribution

It introduces a novel lower bound of n ≥ Ω(k^3) for 3-query LDCs, improving previous bounds and linking LDCs to CSP refutation methods.

Findings

Proves near-cubic lower bound for 3-query LDCs

Develops new techniques based on CSP refutation and Kikuchi matrices

Enhances understanding of LDC limitations in coding theory

Abstract

A code $C \colon \{0,1\}^k \to \{0,1\}^n$ is a $q$-locally decodable code ($q$-LDC) if one can recover any chosen bit $b_i$ of the message $b \in \{0,1\}^k$ with good confidence by randomly querying the encoding $x := C(b)$ on at most $q$ coordinates. Existing constructions of $2$-LDCs achieve $n = \exp(O(k))$, and lower bounds show that this is in fact tight. However, when $q = 3$, far less is known: the best constructions achieve $n = \exp(k^{o(1)})$, while the best known results only show a quadratic lower bound $n \geq \tilde{\Omega}(k^2)$ on the blocklength. In this paper, we prove a near-cubic lower bound of $n \geq \tilde{\Omega}(k^3)$ on the blocklength of $3$-query LDCs. This improves on the best known prior works by a polynomial factor in $k$. Our proof relies on a new connection between LDCs and refuting constraint satisfaction problems with limited randomness. Our quantitative improvement builds on the new techniques for refuting semirandom instances of CSPs developed in [GKM22, HKM23] and, in particular, relies on bounding the spectral norm of appropriate Kikuchi matrices.