Spectra, norms and numerical ranges of generalized quadratic operators

Abstract

A bounded linear operator acting on a Hilbert space is a generalized quadratic operator if it has an operator matrix of the form It reduces to a quadratic operator if d = 0. In this article, spectra, norms and various kinds of numerical ranges of generalized quadratic operators are determined. Some operator inequalities are also obtained. In particular, it is shown that for a given generalized quadratic operator, the rank-k numerical range, the essential numerical range and the q-numerical range are elliptical discs; the c-numerical range is a sum of elliptical discs. The Davis–Wielandt shell is the convex hull of a family of ellipsoids unless the underlying Hilbert space has dimension 2.