Packing, Tiling, Orthogonality and Completeness

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Let Ω⊆Rd be an open set of measure 1. An open set D⊆Rd is called a ‘tight orthogonal packing region’ for Ω if D−D does not intersect the zeros of the Fourier transform of the indicator function of Ω, and D has measure 1. Suppose that Λ is a discrete subset of Rd. The main contribution of this paper is a new way of proving the following result: D tiles Rd when translated at the locations Λ if and only if the set of exponentials EΛ = {exp 2πi〈λ, x〉:λ ∈ Λ} is an orthonormal basis for L2(Ω). (This result has been proved by different methods by Lagarias, Reeds and Wang [9] and, in the case of Ω being the cube, by Iosevich and Pedersen [3]. When Ω is the unit cube in Rd, it is a tight orthogonal packing region of itself.) In our approach, orthogonality of EΛ is viewed as a statement about ‘packing’ Rd with translates of a certain non-negative function and, additionally, we have completeness of EΛ in L2(Ω) if and only if the above-mentioned packing is in fact a tiling. We then formulate the tiling condition in Fourier analytic language, and use this to prove our result. 2000 Mathematics Subject Classification 52C22, 42B99, 11K70.