Roberts orthogonality and the Davis-Wielandt shell

Abstract

Let $\mathcal{A}$ be a unital $C^{\ast }$-algebra with the unit $e$. We consider the elements $a\in \mathcal{A}$ which are Roberts orthogonal to the unit $e$. We obtain a characterization of this orthogonality in terms of the Davis--Wielandt shell of $a$ and show that, for certain classes of elements of $\mathcal{A}$, the Roberts orthogonality of $a$ and $e$ is equivalent to the symmetry of the numerical range of $a$ with respect to the origin.

12 pages, 1 figure