Abstract
An r-cut of a k-uniform hypergraph H is a partition of the vertex set of H into r parts and the size of the cut is the number of edges which have a vertex in each part. A classical result of Edwards says that every m-edge graph has a 2-cut of size \(m{\rm/}2 + {\rm\Omega)}(\sqrt m)\) and this is best possible. That is, there exist cuts which exceed the expected size of a random cut by some multiple of the standard deviation. We study analogues of this and related results in hypergraphs. First, we observe that similarly to graphs, every m-edge k-uniform hypergraph has an r-cut whose size is \({\rm\Omega}(\sqrt m)\) larger than the expected size of a random r-cut. Moreover, in the case where k = 3 and r = 2 this bound is best possible and is attained by Steiner triple systems. Surprisingly, for all other cases (that is, if k ≥ 4 or r ≥ 3), we show that every m-edge k-uniform hypergraph has an r-cut whose size is Ω(m5/9) larger than the expected size of a random r-cut. This is a significant difference in behaviour, since the amount by which the size of the largest cut exceeds the expected size of a random cut is now considerably larger than the standard deviation.
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Research supported by a Royal Society University Research Fellowship and by ERC Starting Grant 676632.
Research supported by a Packard Fellowship and by NSF Career Award DMS-1352121.
This research was done while the author was working at ETH Zurich.
Research supported in part by SNSF grant 200021-175573.
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Conlon, D., Fox, J., Kwan, M. et al. Hypergraph cuts above the average. Isr. J. Math. 233, 67–111 (2019). https://doi.org/10.1007/s11856-019-1897-z
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DOI: https://doi.org/10.1007/s11856-019-1897-z