Exponential bases for partitions of intervals (2109.04441v1)

Abstract: For a partition of $[0,1]$ into intervals $I_1,\ldots,I_n$ we prove the existence of a partition of $\mathbb{Z}$ into $\Lambda_1,\ldots, \Lambda_n$ such that the complex exponential functions with frequencies in $ \Lambda_k$ form a Riesz basis for $L^2(I_k)$, and furthermore, that for any $J\subseteq{1,\,2,\,\dots,\,n}$, the exponential functions with frequencies in $ \bigcup_{j\in J}\Lambda_j$ form a Riesz basis for $L^2(I)$ for any interval $I$ with length $|I|=\sum_{j\in J}|I_j|$. The construction extends to infinite partitions of $[0,1]$, but with size limitations on the subsets $J\subseteq \mathbb{Z}$; it combines the ergodic properties of subsequences of $\mathbb{Z}$ known as Beatty-Fraenkel sequences with a theorem of Avdonin on exponential Riesz bases.