Generalized $A$-numerical radius of operators and related inequalities

Abstract

Let $A$ be a non-zero positive bounded linear operator on a complex Hilbert space $(\mathcal{H},\langle\cdot,\cdot\rangle)$. Let $\omega_A(T)$ denote the $A$-numerical radius of an operator $T$ acting on the semi-Hilbert space $(\mathcal{H},\langle\cdot,\cdot\rangle_A)$, where $\langle x, y\rangle_{A} :=\langle Ax, y\rangle$ for all $x,y\in \mathcal{H}$. Let $N_A(\cdot)$ be a seminorm on the algebra of all $A$-bounded operators acting on $\mathcal{H}$ and let $T$ be an operator which admits $A$-adjoint. Then, we define the generalized $A$-numerical radius as $$\omega_{N_A}(T)=\displaystyle{\sup_{\theta \in \mathbb{R}}}\; N_A\left(\frac{e^{i\theta}T+e^{-i\theta}T^{\sharp_A}}{2}\right),$$ where $T^{\sharp_A}$ denotes a distinguished $A$-adjoint of $T$. We develop several generalized $A$-numerical radius inequalities from which follows the existing numerical radius and $A$-numerical radius inequalities. We also obtain bounds for generalized $A$-numerical radius of sum and product of operators. Finally, we study $\omega_{N_A}(\cdot)$ in the setting of two particular seminorms $N_A(\cdot)$.