Maximal norm Hankel operators

Abstract

A Hankel operator $\mathbf{H}_\varphi$ on the Hardy space $H^2$ of the unit circle with analytic symbol $\varphi$ has minimal norm if $\|\mathbf{H}_\varphi\|=\|\varphi \|_2$ and maximal norm if $\|\mathbf{H}_\varphi\| = \|\varphi\|_\infty$. The Hankel operator $\mathbf{H}_\varphi$ has both minimal and maximal norm if and only if $|\varphi|$ is constant almost everywhere on the unit circle or, equivalently, if and only if $\varphi$ is a constant multiple of an inner function. We show that if $\mathbf{H}_\varphi$ is norm-attaining and has maximal norm, then $\mathbf{H}_\varphi$ has minimal norm. If $|\varphi|$ is continuous but not constant, then $\mathbf{H}_\varphi$ has maximal norm if and only if the set at which $|\varphi|=\|\varphi\|_{\infty}$ has nonempty intersection with the spectrum of the inner factor of $\varphi$. We obtain further results illustrating that the case of maximal norm is in general related to "irregular" behavior of $\log |\varphi|$ or the argument of $\varphi$ near a "maximum point" of $|\varphi|$. The role of certain positive functions coined apical Helson--Szeg\H{o} weights is discussed in the former context.