Abstract
By prior work, it is known that any distributed graph algorithm that finds a maximal matching requires \(\varOmega (\log ^* n)\) communication rounds, while it is possible to find a maximal fractional matching in O(1) rounds in bounded-degree graphs. However, all prior O(1)-round algorithms for maximal fractional matching use arbitrarily fine-grained fractional values. In particular, none of them is able to find a half-integral solution, using only values from \(\{0, \frac{1}{2}, 1\}\). We show that the use of fine-grained fractional values is necessary, and moreover we give a complete characterization on exactly how small values are needed: if we consider maximal fractional matching in graphs of maximum degree \(\varDelta = 2d\), and any distributed graph algorithm with round complexity \(T(\varDelta )\) that only depends on \(\varDelta \) and is independent of n, we show that the algorithm has to use fractional values with a denominator at least \(2^d\). We give a new algorithm that shows that this is also sufficient.
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Acknowledgements
This work was supported in part by the Academy of Finland, Grant 333837. We would like to thank the anonymous reviewers for their helpful feedback, and the members of Aalto Distributed Algorithms group for discussions.
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Dahal, S., Suomela, J. (2023). Distributed Half-Integral Matching and Beyond. In: Rajsbaum, S., Balliu, A., Daymude, J.J., Olivetti, D. (eds) Structural Information and Communication Complexity. SIROCCO 2023. Lecture Notes in Computer Science, vol 13892. Springer, Cham. https://doi.org/10.1007/978-3-031-32733-9_15
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