On Fuglede's Conjecture and the Existence of Universal Spectra

Recent methods developed by Tao 18], Kolountzakis and Matolcsi 7] have led to counterexamples to Fugelde’s Spectral Set Conjecture in both directions. Namely, in Tao produced a spectral set which is not a tile, while Kolountzakis and Matolcsi showed an example of a nonspectral tile. In search of lower dimensional nonspectral tiles we were led to investigate the Universal Spectrum Conjecture (USC) of Lagarias and Wang 14]. In particular, we prove here that the USC and the "tile → spectral" direction of Fuglede’s conjecture are equivalent in any dimensions. Also, we show by an example that the sufficient condition of Lagarias and Szabó 13] for the existence of universal spectra is not necessary. This fact causes considerable difficulties in producing lower dimensional examples of tiles which have no spectra. We overcome these difficulties by invoking some ideas of Révész and Farkas 2], and obtain nonspectral tiles in . Fuglede’s conjecture and the Universal Spectrum Conjecture remains open in 1 and 2 dimensions. The one-dimensional case is closely related to a number theoretical conjecture on tilings by Coven and Meyerowitz 1].