Norm and Minimum Attainment of Foguel-Type Operators

Abstract

In this article, we study Foguel-type operators \EuScript{F}=\begin{bmatrix} V_1^{*} & T\\ 0 & V_2\end{bmatrix} on $H_1\oplus H_2$, where each $H_j$ is a separable Hilbert space, $V_j\in \mathcal{B}(H_j)$ is an isometry for $j=1,2$, and $T\in \mathcal{B}(H_2,H_1)$. We examine how the norm and minimum attainment properties of \EuScript{F} depend on the corner entry $T$ and on the defect spaces of the isometries. The norm formula \|\EuScript{F}\|=\frac{\|T\|+\sqrt{\|T\|^{2}+4}}{2} holds for every $T$ when $V_1,V_2$ are isometries, and more generally when $V_2$ is a contraction that is isometric on the range of $T^{*}$. Moreover, \EuScript{F} attains its norm if and only if $T$ does. We characterize the positive $2\times 2$ block operator matrices that are absolutely norm attaining and absolutely minimum attaining, and use this to prove that \EuScript{F} is absolutely norm attaining if and only if it is absolutely minimum attaining if and only if $\dim N(V_1^{*})<\infty$ and $T$ has finite rank. The closure of the absolutely norm attaining class is described by $\dim N(V_1^{*})<\infty$ and compactness of $T$. Further, we establish that the reduced minimum modulus is γ(\EuScript{F})=\frac{\sqrt{\|T\|^{2}+4}-\|T\|}{2}=\frac{1}{\|\EuScript{F}\|}, and \EuScript{F} attains it if and only if $T$ is norm attaining. We also show that \EuScript{F} is Fredholm if and only if $\dim N(V_j^{*})<\infty$ for $j=1,2$, with index $\dim N(V_1^{*})-\dim N(V_2^{*})$, and we give its Moore–Penrose inverse. Finally, for $V_1=V_2=T_z$ and more generally $V_j=T_{θ_j}$ with $θ_j$ inner, and Toeplitz or Hankel corner entries, the results become explicit conditions on the symbol.