The $L^2$-Cartesian Numerical Radii of Hilbert Space Operators

Abstract

Let $\mathcal{H}$ be a complex Hilbert space, and let $T=H+iK\in \mathcal{B}(\mathcal{H})$ be the Cartesian decomposition of $T$, where $H=H^{\ast }$ and $K=K^{\ast }$. We introduce the $L^2$-Cartesian numerical radius and study its basic properties. We compare it with the classical numerical radius $\omega (T)$. In particular, we establish the sharp estimate $ \frac{1}{\sqrt{2}}ω(T)\le \wcart{2}(T)\le ω(T), $ and establish lower and upper bounds in terms of the Cartesian parts $H,K$. We also give an equality characterization and explicit sharpness examples. As a consequence, we obtain $\frac{1}{4}\left\|T^{*}T+TT^{*}\right\|\le \wcart{2}(T)^2\le ω(T)^2. $ More generally, we study the $L^p$-Cartesian numerical radius and prove the sharp comparison $ c_p\,ω(T)\le \wcart{p}(T)\le ω(T),\qquad 1\le p<\infty. $ Finally, we present a block-operator analogue of the classical refinement of the triangle inequality.