$q$-numerical radius of rank-one operators and the generalized Buzano inequality

Abstract

Here, we study the $q$-numerical radius of rank-one operators on a Hilbert space $\mathcal{H}$. More precisely, for $q\in [0,1]$ and $a,b\in \mathcal{H}$, we establish the formula \[ ω_q(a \otimes b) = \frac{1}{2}\left(\|a\|\|b\| + q|\langle a, b \rangle| + \sqrt{1-q^2}\sqrt{\|a\|^2\|b\|^2 - |\langle a, b \rangle|^2}\right), \] which represents a generalization of the well-known formula for the numerical radius of a rank-one operator in a Hilbert space, obtained by setting $q=1$. As a corollary, we also derive a generalization of the classical Buzano inequality.