A note on Sidon sets in bounded orthonormal systems

Abstract

We give a simple example of an $n$-tuple of orthonormal elements in $L_2$ (actually martingale differences) bounded by a fixed constant, and hence subgaussian with a fixed constant but that are Sidon only with constant $\approx \sqrt n$. This is optimal. The first example of this kind was given by Bourgain and Lewko, but with constant $\approx \sqrt {\log n}$. We also include the analogous $n\times n$-matrix valued example, for which the optimal constant is $\approx n$. We deduce from our example that there are two $n$-tuples each Sidon with constant 1, lying in orthogonal linear subspaces and such that their union is Sidon only with constant $\approx \sqrt n$. This is again asymptotically optimal. We show that any martingale difference sequence with values in $[-1,1]$ is "dominated" in a natural sense (related to our results) by any sequence of independent, identically distributed, symmetric $\{-1,1\}$-valued variables (e.g. the Rademacher functions). We include a self-contained proof that any sequence $(\varphi_n)$ that is the union of two Sidon sequences lying in orthogonal subspaces is such that $(\varphi_n\otimes\varphi_n \otimes\varphi_n\otimes\varphi_n)$ is Sidon.