In this paper, we consider saturation problems related to the celebrated Erdős-Szekeres convex polygon problem. For each n ≥ 7, we construct a planar point set of size (7/8) ⋅ 2^{n-2} which is saturated for convex n-gons. That is, the set contains no n points in convex position while the addition of any new point creates such a configuration. This demonstrates that the saturation number is smaller than the Ramsey number for the Erdős-Szekeres problem. The proof also shows that the original Erdős-Szekeres construction is indeed saturated. Our construction is based on a similar improvement for the saturation version of the cups-versus-caps theorem. Moreover, we consider the generalization of the cups-versus-caps theorem to monotone paths in ordered hypergraphs. In contrast to the geometric setting, we show that this abstract saturation number is always equal to the corresponding Ramsey number.
@InProceedings{damasdi_et_al:LIPIcs.SoCG.2024.46, author = {Dam\'{a}sdi, G\'{a}bor and Dong, Zichao and Scheucher, Manfred and Zeng, Ji}, title = {{Saturation Results Around the Erd\H{o}s-Szekeres Problem}}, booktitle = {40th International Symposium on Computational Geometry (SoCG 2024)}, pages = {46:1--46:14}, 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.2024.46}, URN = {urn:nbn:de:0030-drops-199919}, doi = {10.4230/LIPIcs.SoCG.2024.46}, annote = {Keywords: Convex polygon, Cups-versus-caps, Monotone path, Saturation problem} }
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