โ Scribed by Kuhn, Fabian; Wattenhofer, Roger
No coin nor oath required. For personal study only.
Coloring the nodes of a graph with a small number of colors is one of the most fundamental problems in theoretical computer science. In this paper, we study graph coloring in a distributed setting. Processors of a distributed system are nodes of an undirected graph G. There is an edge between two nodes whenever the corresponding processors can directly communicate with each other. We assume that distributed coloring algorithms start with an initial m-coloring of G. In the paper, we prove new strong lower bounds for two special kinds of coloring algorithms. For algorithms which run for a single communication round-i.e., every node of the network can only send its initial color to all its neighbors-, we show that the number of colors of the computed coloring has to be at least โฆ(โ 2 / log 2 โ + log log m). If such one-round algorithms are iteratively applied to reduce the number of colors step-by-step, we prove a time lower bound of โฆ(โ/ log 2 โ + log * m) to obtain an O(โ)coloring. The best previous lower bounds for the two types of algorithms are โฆ(log log m) and โฆ(log * m), respectively.
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