In this paper, we prove that it is undecidable whether a set of two polycubes can tile Z³ by translation. The proof involves a new technique that allows us to simulate two disconnected polycubes with two connected polycubes. By expanding this technique to higher dimensions, we also prove that a set of disconnected tiles in Zⁿ can be simulated by the same number of connected tiles in Zⁿ for n 3.
Y. Kim (Fri,) studied this question.