We consider linearly independent families of Hermitian matrices {A1, . . . , Am} so thatWk(A) is convex. It is shown that m can reach the upper bound 2k(n− k) + 1. A key idea in our study is relating the convexity of Wk(A) to the problem of constructing rank k orthogonal projections under linear constraints determined by A. The techniques are extended to study the convexity of other generalized...

The numerical range W (A) of an n×n matrix A is the collection of complex numbers of the form x∗Ax, where x ∈ C is a unit vector. It can be viewed as a “picture” of A containing useful information of A. Even if the matrix A is not known explicitly, the “picture” W (A) would allow one to “see” many properties of the matrix. For example, the numerical range can be used to locate eigenvalues, dedu...

The numerical range of a matrix is studied geometrically via the cone of positive semidefinite matrices (or semidefinite cone for short). In particular it is shown that the feasible set of a two-dimensional linear matrix inequality (LMI), an affine section of the semidefinite cone, is always dual to the numerical range of a matrix, which is therefore an affine projection of the semidefinite con...

In this paper, we collect a few fairly well known facts about the numerical range and assemble them into an effective algorithm for computing the numerical range of an n× n matrix. Particular attention is paid to the case in which a line segment is embedded in the boundary of the numerical range, a case in which multiplicity is present. The result is a parametrization of the curve that forms th...

We offer an almost self-contained development of Perron–Frobenius type results for the numerical range of an (irreducible) nonnegative matrix, rederiving and completing the previous work of Issos, Nylen and Tam, and Tam and Yang on this topic. We solve the open problem of characterizing nonnegative matrices whose numerical ranges are regular convex polygons with center at the origin. Some relat...

We define the complete numerical radius norm for homomorphisms from any operator algebra into B(H), and show that this norm can be computed explicitly in terms of the completely bounded norm. This is used to show that if K is a complete Cspectral set for an operator T , then it is a complete M -numerical radius set, where M = 12 (C + C −1). In particular, in view of Crouzeix’s theorem, there is...

We show that only the segment borders have to be taken into account as cut point candidates in searching for the optimal multisplit of a numerical value rangewith respect to convex attribute evaluation functions. Segment borders can be found efficiently in a linear-time preprocessing step. For strictly convex evaluation functions inspecting all segment borders is also necessary. With Training S...