Greenfeld and Lev J. Anal. Math., 16 (2020), 409–441 conjectured that the Cartesian product of two sets A A and B B is spectral if and only if A A and B B are spectral. We construct a counterexample to this conjecture using the existence of a tile that has no spectra.
Gábor Somlai (Thu,) studied this question.
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