We prove that the following problem is co-RE-complete and thus undecidable: given three simple polygons, is there a tiling of the plane where every tile is an isometry of one of the three polygons (either allowing or forbidding reflections)? This result improves on the best previous construction which requires five polygons.
@InProceedings{demaine_et_al:LIPIcs.SoCG.2025.39, author = {Demaine, Erik D. and Langerman, Stefan}, title = {{Tiling with Three Polygons Is Undecidable}}, booktitle = {41st International Symposium on Computational Geometry (SoCG 2025)}, pages = {39:1--39:16}, 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.2025.39}, URN = {urn:nbn:de:0030-drops-231913}, doi = {10.4230/LIPIcs.SoCG.2025.39}, annote = {Keywords: plane tilings, polygons, undecidability, co-RE} }
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