## Abstract

We consider graph coloring and related problems in the distributed message-passing model. Locally-iterative algorithms are especially important in this setting. These are algorithms in which each vertex decides about its next color only as a function of the current colors in its 1-hop-neighborhood. In STOC'93 Szegedy and Vishwanathan showed that any locally-iterative Δ+ 1-coloring algorithm requires ω (Δlog Δ+ log ∗ n) rounds, unless there exists "a very special type of coloring that can be very efficiently reduced"[44]. No such special coloring has been found since then. This led researchers to believe that Szegedy-Vishwanathan barrier is an inherent limitation for locally-iterative algorithms and to explore other approaches to the coloring problem [2, 3, 19, 32]. The latter gave rise to faster algorithms, but their heavy machinery that is of non-locally-iterative nature made them far less suitable to various settings. In this article, we obtain the aforementioned special type of coloring. Specifically, we devise a locally-iterative Δ+ 1-coloring algorithm with running time O(Δ+ log ∗ n), i.e., below Szegedy-Vishwanathan barrier. This demonstrates that this barrier is not an inherent limitation for locally-iterative algorithms. As a result, we also achieve significant improvements for dynamic, self-stabilizing, and bandwidth-restricted settings. This includes the following results:We obtain self-stabilizing distributed algorithms for Δ+ 1-vertex-coloring, (2Δ- 1)-edge-coloring, maximal independent set, and maximal matching with O(Δ+ log ∗ n) time. This significantly improves previously known results that have O(n) or larger running times [23].We devise a (2Δ- 1)-edge-coloring algorithm in the CONGEST model with O(Δ+ log ∗ n) time and O(δ)-edge-coloring in the Bit-Round model with O(Δ+ log n) time. The factors of log ∗ n and log n are unavoidable in the CONGEST and Bit-Round models, respectively. Previously known algorithms had superlinear dependency on Δfor (2Δ- 1)-edge-coloring in these models.We obtain an arbdefective coloring algorithm with running time O( Δ+ log ∗ n). Such a coloring is not necessarily proper, but has certain helpful properties. We employ it to compute a proper (1 + ϵ)δ-coloring within O( Δ+ log ∗ n) time and Δ+ 1-coloring within O( Δlog Δlog ∗ Δ+ log ∗ n) time. This improves the recent state-of-the-art bounds of Barenboim from PODC'15 [2] and Fraigniaud et al. from FOCS'16 [19] by polylogarithmic factors.Our algorithms are applicable to the SET-LOCAL model [25] (also known as the weak LOCAL model). In this model a relatively strong lower bound of ω (Δ1/3) is known for Δ+ 1-coloring. However, most of the coloring algorithms do not work in this model. (In Reference [25] only Linial's O(Δ2)-time algorithm and Kuhn-Wattenhofer O(Δlog δ)-time algorithms are shown to work in it.) We obtain the first linear-in-ΔΔ+ 1-coloring algorithms that work also in this model.

Original language | English |
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Article number | 5 |

Pages (from-to) | 5:1-5:26 |

Number of pages | 26 |

Journal | Journal of the ACM |

Volume | 69 |

Issue number | 1 |

DOIs | |

State | Published - Feb 2022 |

### Bibliographical note

Funding Information:This research has been supported by Israel Science Foundation grant 724/15. Authors’ addresses: L. Barenboim and U. Goldenberg, The Open University of Israel, Department of Mathematics and Computer Science, 1 University Road, P. O. Box 808, Raanana 43107; emails: leonidb@openu.ac.il, uri.goldenberg@gmail.com; M. Elkin, Ben-Gurion University of the Negev, Department of Computer Science, P.O.Box 653 Beer-Sheva 8410501; email: elkinm@cs.bgu.ac.il. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. © 2021 Association for Computing Machinery. 0004-5411/2021/12-ART5 $15.00 https://doi.org/10.1145/3486625

Publisher Copyright:

© 2021 Association for Computing Machinery.

## Keywords

- congest networks
- self-stabilization
- Symmetry-breaking