Numerical radius inequalities of bounded linear operators and $(\alpha,\beta)$-normal operators

Abstract

We obtain various upper bounds for the numerical radius $w(T)$ of a bounded linear operator $T$ defined on a complex Hilbert space $\mathcal{H}$, by developing the upper bounds for the $\alpha$-norm of $T$, which is defined as $\|T\|_{\alpha}= \sup \left\{ \sqrt{\alpha |\langle Tx,x \rangle|^2+ (1-\alpha)\|Tx\|^2 } : x\in \mathcal{H}, \|x\|=1 \right\}$ for $ 0\leq \alpha \leq 1 $. Further, we prove that \begin{eqnarray*} w(T) &\leq & \sqrt{\left( \min_{\alpha \in [0,1]}\left\| \alpha |T|+(1-\alpha)|T^*| \right\| \right) \|T\|} \,\,\,\, \leq \,\, \,\, \|T\|. \end{eqnarray*} For $0\leq \alpha \leq 1 \leq \beta,$ the operator $T$ is called $(\alpha,\beta)$-normal if $\alpha^2 T^*T\leq TT^*\leq \beta^2 T^*T$ holds. Note that every invertible operator is an $(\alpha,\beta)$-normal operator for suitable values of $\alpha$ and $\beta$. Among other lower bound for the numerical radius of an $(\alpha,\beta)$-normal operator $T$, we show that \begin{eqnarray*} w(T) &\geq & \sqrt{\max \left\{ 1+\alpha^2, 1+\frac{1}{\beta^2}\right\} \frac{\|T\|^2}{4}+ \frac {\left| \|\Re(T)\|^2-\|\Im(T)\|^2 \right|}2} &\geq & \max \left\{ \sqrt{1+\alpha^2}, \sqrt{1+\frac{1}{\beta^2}} \right\} \frac{\|T\|}{2} & > & \frac{\|T\|}2, \end{eqnarray*} where $\Re(T)$ and $\Im(T)$ are the real part and imaginary part of $T$, respectively.