PODC 2019 · 38th ACM Symposium on Principles of Distributed Computing, Toronto, Canada, July–August 2019 · doi:10.1145/3293611.3331605

A graph is *weakly 2-colored* if the nodes are labeled with colors black and white such that each black node is adjacent to at least one white node and vice versa. In this work we study the distributed computational complexity of weak 2-coloring in the standard LOCAL model of distributed computing, and how it is related to the distributed computational complexity of other graph problems.

First, we show that weak 2-coloring is a *minimal* distributed symmetry-breaking problem for regular even-degree trees and high-girth graphs: if there is any non-trivial *locally checkable labeling problem* that is solvable in $o(\log^* n)$ rounds with a distributed graph algorithm in the middle of a regular even-degree tree, then weak 2-coloring is also solvable in $o(\log^* n)$ rounds there.

Second, we prove a *tight lower bound* of $\Omega(\log^* n)$ for the distributed computational complexity of weak 2-coloring in regular trees; previously only a lower bound of $\Omega(\log \log^* n)$ was known. By minimality, the same lower bound holds for any non-trivial locally checkable problem inside regular even-degree trees.

Peter Robinson and Faith Ellen (Eds.): *Proceedings of the 2019 ACM Symposium on Principles of Distributed Computing*, pages 369–378, ACM Press, New York, 2019

ISBN 978-1-4503-6217-7