Undecidability of Translational Tiling with Three Tiles

Is there a fixed dimension $n$ such that translational tiling of ${\mathbb{Z}}^n$ with a monotile is undecidable? Several recent results support a positive answer to this question. Greenfeld and Tao disprove the periodic tiling conjecture by showing that an aperiodic monotile exists in sufficiently high dimension $n$ [Ann. Math. 200(2024), 301-363]. In another paper [to appear in J. Eur. Math. Soc.], they also show that if the dimension $n$ is part of the input, then the translational tiling for subsets of ${\mathbb{Z}}^n$ with one tile is undecidable. These two results are very strong pieces of evidence for the conjecture that translational tiling of ${\mathbb{Z}}^n$ with a monotile is undecidable, for some fixed $n$. This paper gives another supportive result for this conjecture by showing that translational tiling of the $4$-dimensional space with a set of three connected tiles is undecidable.