It is shown that the result of Tso-Wu on the elliptical shape of the numerical range of quadratic operators holds also for the essential numerical range. The latter is described quantitatively, and based on that sufficient conditions are established under which the c-numerical range also is an ellipse. Several examples are considered, including singular integral operators with the Cauchy kernel...

It is shown that each contraction A on a Hilbert space H, with A + A I for some 2 R, has a unitary dilation U on H H satisfying U + U I. This is used to settle a conjecture of Halmos in the aarmative: The closure of the numerical range of each contraction A is the intersection of the closures of the numerical ranges of all unitary dilations of A. By means of the duality theory of completely pos...

A criterion for the numerical range of a linear operator acting in a Krein space to be a two-component hyperbolical disc is given, using the concept of support function. A characterization of the Krein space numerical range as a union of hyperbolical discs is obtained by a reduction to the two-dimensional case. We revisit a famous result of Ando concerning the inclusion relation W (A) ⊆ W (B) o...

Which convex subsets of C are the numerical range W (A) of some matrix A? This paper gives a precise characterization of these sets. In addition to this we show that for any A there exists a symmetric B of the same size such that W (A) = W (B) thereby settling an open question from [2]. Mathematics Subject Classification (2000). Primary 47A12.

the rank-k numerical range has a close connection to the construction of quantum error correction code for a noisy quantum channel. for noisy quantum channel, a quantum error correcting code of dimension k exists if and only if the associated joint rank-k numerical range is non-empty. in this paper the notion of joint rank-k numerical range is generalized and some statements of [2011, generaliz...