New Hardness Results for the LOCAL Model via a Simple Self-Reduction

TL;DR

This paper simplifies the round elimination via self-reduction technique to establish new, tighter lower bounds on the round complexity of distributed algorithms for maximal matching, b-matching, and edge coloring problems in the LOCAL model.

Contribution

It introduces a simplified version of the round elimination technique and applies it to derive new lower bounds for multiple graph problems in the LOCAL model.

Findings

Established lower bounds for maximal b-matching algorithms.

Provided simplified proofs for existing bounds on maximal matching.

Derived lower bounds for edge coloring with limited colors.

Abstract

Very recently, Khoury and Schild [FOCS 2025] showed that any randomized LOCAL algorithm that solves maximal matching requires $\Omega(\min\{\log \Delta, \log_\Delta n\})$ rounds, where $n$ is the number of nodes in the graph and $\Delta$ is the maximum degree. This result is shown through a new technique, called round elimination via self-reduction. The lower bound proof is beautiful and presents very nice ideas. However, it spans more than 25 pages of technical details, and hence it is hard to digest and generalize to other problems. Historically, the simplification of proofs and techniques has marked an important turning point in our understanding of the complexity of graph problems. Our paper makes a step forward towards this direction, and provides the following contributions. 1. We present a short and simplified version of the round elimination via self-reduction technique. The simplification of this technique enables us to obtain the following two hardness results. 2. We show that any randomized LOCAL algorithm that solves the maximal $b$-matching problem requires $\Omega(\min\{\log_{1+b}\Delta, \log_\Delta n\})$ and $\Omega(\sqrt{\log_{1+b} n})$ rounds. We recall that the $b$-matching problem is a generalization of the matching problem where each vertex can have up to $b$ incident edges in the matching. As a corollary, for $b=1$, we obtain a short proof for the maximal matching lower bound shown by Khoury and Schild. 3. Finally, we show that any randomized LOCAL algorithm that properly colors the edges of a graph with $\Delta + k$ colors requires $\Omega(\min\{\log \Delta, \log_\Delta n\})$ and $\Omega(\sqrt{\log n})$ rounds, for any $k\le \Delta^{1-\varepsilon}$ and any constant $\varepsilon > 0$.