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We study the discrepancy of the following communication problem. Alice receives a halfplane, and Bob receives a point in the plane, and their goal is to determine whether Bob’s point belongs to Alice’s halfplane. This communication task corresponds to determining whether x₁y₁+y₂ ≥ x₂, where the first player knows (x₁,x₂) and the second player knows (y₁,y₂).
Denoting n = m³, we show that when the inputs are chosen from [m] × [m²], the communication discrepancy of the above problem is O(n^{-1/6} log^{3/2} n).
On the other hand, through the connections to the notion of hereditary discrepancy by Matoušek, Nikolov, and Tawler (IMRN 2020) and a classical result of Matoušek (Discrete Comput. Geom. 1995), we show that the communication discrepancy of every set of n points and n halfplanes is at least Ω(n^{-1/4} log^{-1} n).
@InProceedings{ahmed_et_al:LIPIcs.SoCG.2024.5,
author = {Ahmed, Manasseh and Cheung, Tsun-Ming and Hatami, Hamed and Sareen, Kusha},
title = {{Communication Complexity and Discrepancy of Halfplanes}},
booktitle = {40th International Symposium on Computational Geometry (SoCG 2024)},
pages = {5:1--5:17},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-316-4},
ISSN = {1868-8969},
year = {2024},
volume = {293},
editor = {Mulzer, Wolfgang and Phillips, Jeff M.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2024.5},
URN = {urn:nbn:de:0030-drops-199504},
doi = {10.4230/LIPIcs.SoCG.2024.5},
annote = {Keywords: Randomized communication complexity, Discrepancy theory, factorization norm}
}