A generalization of the 0-numerical range
Rajna Rajić · Glasnik Matematicki · 2004
Let H be a complex Hilbert space.Given a bounded linear operator A on H, we describe the set R n (A) = {V * AW : V, W :It is shown that the closed matricial convex hull of R n (A) is a closed ball of radius min{ A -λI : λ ∈ C} centered at the origin.