The joint essential numerical range of operators: convexity and related results
Chi-Kwong Li, Yiu‐Tung Poon · Studia Mathematica · 2009
Let $W({\bf A})$ and $W_{\rm e}({\bf A})$ be the joint numerical range and the joint essential numerical range of an $m$-tuple of self-adjoint operators ${\bf A} = (A_1, \dots, A_m)$ acting on an infinite-dimensional Hilbert space. It is shown that $W_{\