Orthogonal exponentials, translations, and Bohr completions

Abstract

We are concerned with an harmonic analysis in Hilbert spaces L 2 ( μ ) , where μ is a probability measure on R n . The unifying question is the presence of families of orthogonal (complex) exponentials e λ ( x ) = exp ( 2 π i λ x ) in L 2 ( μ ) . This question in turn is connected to the existence of a natural embedding of L 2 ( μ ) into an L 2 -space of Bohr almost periodic functions on R n . In particular we explore when L 2 ( μ ) contains an orthogonal basis of e λ functions, for λ in a suitable discrete subset in R n ; i.e, when the measure μ is spectral. We give a new characterization of finite spectral sets in terms of the existence of a group of local translation. We also consider measures μ that arise as fixed points (in the sense of Hutchinson) of iterated function systems (IFSs), and we specialize to the case when the function system in the IFS consists of affine and contractive mappings in R n . We show in this case that if μ is then assumed spectral then its partitions induced by the IFS at hand have zero overlap measured in μ. This solves part of the Łaba–Wang conjecture. As an application of the new non-overlap result, we solve the spectral-pair problem for Bernoulli convolutions advancing in this way a theorem of Ka-Sing Lau. In addition we present a new perspective on spectral measures and orthogonal Fourier exponentials via the Bohr compactification.