Improved inequalities for the numerical radius via Cartesian decomposition

Abstract: We develop various lower bounds for the numerical radius $w(A)$ of a bounded linear operator $A$ defined on a complex Hilbert space, which improve the existing inequality $w^2(A)\geq \frac{1}{4}|A^A+AA^|$. In particular, for $r\geq 1$, we show that \begin{eqnarray*}\frac{1}{4}|A^A+AA^| \leq\frac{1}{2} \left( \frac{1}{2}|\Re(A)+\Im(A)|^{2r}+\frac{1}{2}|\Re(A)-\Im(A)|^{2r}\right)^{\frac{1}{r}} \leq w^{2}(A),\end{eqnarray*} where $\Re(A)$ and $\Im(A)$ are the real and imaginary parts of $A$, respectively. Furthermore, we obtain upper bounds for $w^2(A)$ refining the well-known upper bound $w^2(A)\leq \frac{1}{2} \left(w(A^2)+|A|^2\right)$. Separate complete characterizations for $w(A)=\frac{|A|}{2}$ and $w(A)=\frac{1}{2}\sqrt{|A^A+AA^|}$ are also given.