On the numerical radius of weighted shifts on $\ell^{2}$ as a norm on $\ell^{\infty}$ and connections to Banach limits

Abstract

We investigate the numerical radius of weighted shifts on $\ell^{2}$ as a norm on $\ell^{\infty}$. Along the way, we prove that the largest value a Banach limit can attain on the sequence of absolute values of a bounded sequences is larger or equal to the spectral radius and smaller or equal to the numerical radius of the corresponding weighted shift. Further, we compare the operator norms on $\ell^{\infty}$ with respect to the uniform norm and the numerical radius of weighted shifts as a norm. We show that they generally differ, but for multiplication operators and weighted shifts on $\ell^{\infty}$, they are both equal to the uniform norm of the corresponding weight. Moreover, we prove that all complex-valued Banach limits satisfy the norm inequality with respect to the numerical radius of weighted shifts and provide an application of our results to the theory of Banach limits.