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We initiate the study of computing shortest non-separating simple closed curves with some given topological properties on non-orientable surfaces. While, for orientable surfaces, any two non-separating simple closed curves are related by a self-homeomorphism of the surface, and computing shortest such curves has been vastly studied, for non-orientable ones the classification of non-separating simple closed curves up to ambient homeomorphism is subtler, depending on whether the curve is one-sided or two-sided, and whether it is orienting or not (whether it cuts the surface into an orientable one).
We prove that computing a shortest orienting (weakly) simple closed curve on a non-orientable combinatorial surface is NP-hard but fixed-parameter tractable in the genus of the surface. In contrast, we can compute a shortest non-separating non-orienting (weakly) simple closed curve with given sidedness in g^{O(1)} ⋅ n log n time, where g is the genus and n the size of the surface.
For these algorithms, we develop tools that can be of independent interest, to compute a variation on canonical systems of loops for non-orientable surfaces based on the computation of an orienting curve, and some covering spaces that are essentially quotients of homology covers.
@InProceedings{bulavka_et_al:LIPIcs.SoCG.2024.28,
author = {Bulavka, Denys and Colin de Verdi\`{e}re, \'{E}ric and Fuladi, Niloufar},
title = {{Computing Shortest Closed Curves on Non-Orientable Surfaces}},
booktitle = {40th International Symposium on Computational Geometry (SoCG 2024)},
pages = {28:1--28:16},
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.2024.28},
URN = {urn:nbn:de:0030-drops-199731},
doi = {10.4230/LIPIcs.SoCG.2024.28},
annote = {Keywords: Surface, Graph, Algorithm, Non-orientable surface}
}