Some Operator Bounds Employing Complex Interpolation Revisited

Abstract: We revisit and extend known bounds on operator-valued functions of the type $$ T_1^{-z} S T_2^{-1+z}, \quad z \in \ol \Sigma = {z\in\bbC\,|\, \Re(z) \in [0,1]}, $$ under various hypotheses on the linear operators $S$ and $T_j$, $j=1,2$. We particularly single out the case of self-adjoint and sectorial operators $T_j$ in some separable complex Hilbert space $\cH_j$, $j=1,2$, and suppose that $S$ (resp., $S^*$) is a densely defined closed operator mapping $\dom(S) \subseteq \cH_1$ into $\cH_2$ (resp., $\dom(S^*) \subseteq \cH_2$ into $\cH_1$), relatively bounded with respect to $T_1$ (resp., $T_2^*$). Using complex interpolation methods, a generalized polar decomposition for $S$, and Heinz's inequality, the bounds we establish lead to inequalities of the following type, \begin{align*} & \big|\ol{T_2^{-x}ST_1^{-1+x}}\big|_{\cB(\cH_1,\cH_2)} \leq N_1 N_2 e^{(\theta_1 + \theta_2) [x(1-x)]^{1/2}} \ & \quad \times \big|ST_1^{-1}\big|_{\cB(\cH_1,\cH_2)}^{1-x} \, \big|S^(T_2^)^{-1}\big|_{\cB(\cH_2,\cH_1)}^{x}, \quad x \in [0,1], \end{align*} assuming that $T_j$ have bounded imaginary powers, that is, for some $N_j\ge 1$ and $\theta_j \ge 0,$ $$ \big|T_j^{is}\big|_{\cB(\cH)} \leq N_j e^{\theta_j |s|}, \quad s \in \bbR, \; j=1,2. $$ We also derive analogous bounds with $\cB(\cH_1,\cH_2)$ replaced by trace ideals, $\cB_p(\cH_1, \cH_2)$, $p \in [1,\infty)$. The methods employed are elementary, predominantly relying on Hadamard's three-lines theorem and Heinz's inequality.