arXiv subjects
Publications and source records attributed to Alexei Poltoratski.
Let $\fre$ be a homogeneous subset of $\bbR$ in the sense of Carleson. Let $μ$ be a finite positive measure on $\bbR$ and $H_μ(x)$ its Hilbert transform. We prove that if $\lim_{t\to\infty} t \abs{\fre\cap\{x\mid\abs{H_μ(x)}>t\}}=0$, then $μ_s(\fre)=0$, where $μ_\s$ is the singular part of $μ$.
We study several related aspects of reflectionless Jacobi matrices. Our first set of results deals with the singular part of reflectionless measures. We then introduce and discuss Lyapunov exponents, density of states measures, and other related quantities in a \textit{general} setting. This is related to the previous material because the density of states measures are reflectionless on certain sets.
We prove that a random function in the Hardy space $H^2$ is a non-cyclic vector for the backward shift operator almost surely. The question of existence of a local pseudocontinuation for a random analytic function is also studied.
If $μ$ is a finite complex measure in the complex plane $\C$ we denote by $C^μ$ its Cauchy integral defined in the sense of principal value. The measure $μ$ is called reflectionless if it is continuous (has no atoms) and $C^μ=0$ at $μ$-almost every point. We show that if $μ$ is reflectionless and its Cauchy maximal function $C^μ_*$ is summable with respect to $|μ|$ then $μ$ is trivial. An example of a reflectionless measure whose maximal function belongs to the "weak" $L^1$ is also constructed, proving that the above result is sharp in its scale. We also give a partial geometric description of the set of reflectionless measures on the line and discuss connections of our results with the notion of sets of finite perimeter in the sense of De Giorgi.