An extremal problem for functions annihilated by a Toeplitz operator

Abstract

For a bounded function $\varphi$ on the unit circle $\mathbb T$, let $T_\varphi$ be the associated Toeplitz operator on the Hardy space $H^2$. Assume that the kernel $$K_2(\varphi):=\{f\in H^2:\,T_\varphi f=0\}$$ is nontrivial. Given a unit-norm function $f$ in $K_2(\varphi)$, we ask whether an identity of the form $|f|^2=\frac12\left(|f_1|^2+|f_2|^2\right)$ may hold a.e. on $\mathbb T$ for some $f_1,f_2\in K_2(\varphi)$, both of norm $1$ and such that $|f_1|\ne|f_2|$ on a set of positive measure. We then show that such a decomposition is possible if and only if either $f$ or $\overline{z\varphi f}$ has a nontrivial inner factor. The proof relies on an intrinsic characterization of the moduli of functions in $K_2(\varphi)$, a result which we also extend to $K_p(\varphi)$ (the kernel of $T_\varphi$ in $H^p$) with $1\le p\le\infty$.