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The Erdős-Anning theorem states that every point set in the Euclidean plane with integer distances must be either collinear or finite. More strongly, for any (non-degenerate) triangle of diameter δ, at most O(δ²) points can have integer distances from all three triangle vertices. We prove the same results for any strictly convex distance function on the plane, and analogous results for every two-dimensional complete Riemannian manifold of bounded genus and for geodesic distance on the boundary of every three-dimensional Euclidean convex set. As a consequence, we resolve a 1983 question of Richard Guy on the equilateral dimension of Riemannian manifolds. Our proofs are based on the properties of additively weighted Voronoi diagrams of these distances.
@InProceedings{eppstein:LIPIcs.SoCG.2025.46,
author = {Eppstein, David},
title = {{Non-Euclidean Erd\H{o}s-Anning Theorems}},
booktitle = {41st International Symposium on Computational Geometry (SoCG 2025)},
pages = {46:1--46:15},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-370-6},
ISSN = {1868-8969},
year = {2025},
volume = {332},
editor = {Aichholzer, Oswin and Wang, Haitao},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2025.46},
URN = {urn:nbn:de:0030-drops-231983},
doi = {10.4230/LIPIcs.SoCG.2025.46},
annote = {Keywords: integer distances, additively weighted Voronoi diagrams, convex distance functions, Riemannian manifolds, geodesic distance}
}