Reverse inequalities for the Berezin number of operators

Abstract

For a bounded linear operator $A$ on a reproducing kernel Hilbert space $\mathcal{H}(\Omega)$, with normalized reproducing kernel $\widehat{k}_{\lambda} = \frac{k_{\lambda}}{\lVert k_{\lambda}\lVert}$, the Berezin symbol, Berezin number and Berezin norm are defined respectively by $\widetilde{A}(\lambda) = \langle A\widehat{k}_{\lambda},\widehat{k}_{\lambda}\rangle$, $ber(A) = \sup_{\lambda\in\Omega}\left|\widetilde{A}(\lambda)\right|$ and $\left\|A\right\|_{ber} = \sup_{\lambda\in\Omega}\left\|A\widehat{k}_{\lambda}\right\|$. A straightforward comparison between these characteristics yields the inequalities $ber(A)\leq\left\|A\right\|_{ber}\leq\lVert A\lVert$. In this paper, we prove further inequalities relating them, and give special care to the corresponding reverse inequalities. In particular, we refine the first one of the above inequalities, namely we prove that \ $ber(A)\leq\left( \left\|A\right\|_{ber}^{2}-\inf_{\lambda\in\Omega}\left\lVert (A-\widetilde{A}(\lambda))\widehat{k}_{\lambda}\right\lVert^{2}\right) ^{\frac{1}{2}}$.