,
Rustam Latypov
,
Yannic Maus
,
Jara Uitto
Creative Commons Attribution 4.0 International license
Given a graph G = (V,E), a β-ruling set is a subset of nodes S ⊆ V that is independent, and each node in V is at distance at most β from some node in S. In this paper, we present almost optimal distributed algorithms for finding 2-ruling sets in the classical LOCAL model. Our main contribution is a randomized algorithm that w.h.p. computes a 2-ruling set on any n-node graph with bounded arboricity in O(log log n) rounds. In fact, the algorithm works up to arboricity O(log log n), improves exponentially over the prior state of the art that can be achieved by combining [Barenboim, Elkin, Pettie, Schneider; JACM'16], [Ghaffari; SODA'16], and [Bisht, Kothapalli and Pemmaraju; PODC'14], and nearly matches the lower bound of Ω(log log n / log log log n) [Balliu, Brandt, Kuhn, Olivetti; FOCS'20]. The domination parameter β = 2 is optimal for algorithms with runtime log^{o(1)}n: on graphs with arboricity 2, there is a lower bound of Ω(√{log n}) rounds for MIS (i.e., β = 1) [Khoury, Schild; FOCS'25].
Additionally, we obtain improved algorithms for larger arboricity. For general graphs with arboricity α, we present a randomized algorithm that computes a 2-ruling set in Õ(log^{5/8} α +log^{5/3} log n) rounds. This improves exponentially over the state of the art for a large range of non-constant arboricity.
Our techniques extend beyond distributed computing. We present an O(log log log n)-round algorithm in the low-space Massively Parallel Computation (MPC) model that w.h.p. computes a 2-ruling set on any graph with arboricity up to 2^poly(log log n), improving exponentially over the state of the art from [Kothapalli, Pai, Pemmaraju; FSTTCS'20] combined with [Fischer, Giliberti, Grunau; SPAA'23].
@InProceedings{baumecker_et_al:LIPIcs.DISC.2026.15,
author = {Baumecker, Malte and Latypov, Rustam and Maus, Yannic and Uitto, Jara},
title = {{Near-Optimal Distributed 2-Ruling Sets on Graphs with Low Arboricity}},
booktitle = {40th International Symposium on Distributed Computing (DISC 2026)},
pages = {15:1--15:25},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-454-3},
ISSN = {1868-8969},
year = {2026},
volume = {397},
editor = {Moses Jr., William K. and Censor-Hillel, Keren},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2026.15},
URN = {urn:nbn:de:0030-drops-281213},
doi = {10.4230/LIPIcs.DISC.2026.15},
annote = {Keywords: 2-ruling set, LOCAL model, CONGEST model, MPC model}
}