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A Lower Bound on Opaque Sets Kawamura, Akitoshi Moriyama, Sonoko Otachi, Yota Pach, János barriers; Cauchy-Crofton formula; lower bound; opaque sets It is proved that the total length of any set of countably many rectifiable curves, whose union meets all straight lines that intersect the unit square U, is at least 2.00002. This is the first improvement on the lower bound of 2 by Jones in 1964. A similar bound is proved for all convex sets U other than a triangle. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Akitoshi Kawamura and Sonoko Moriyama and Yota Otachi and János Pach 2016 Is Part Of LIPIcs, Volume 51, 32nd International Symposium on Computational Geometry (SoCG 2016) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.SoCG.2016.46 urn:nbn:de:0030-drops-59386 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2016.46 eng https://creativecommons.org/licenses/by/3.0/legalcode