Erik D. Demaine and Stefan Langerman, “Tiling with Three Polygons is Undecidable”, in Proceedings of the 41st International Symposium on Computational Geometry (SoCG 2025), Kanazawa, Japan, June 23–27, 2025, 39:1–39:16.
Abstract:
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.