A GENERALIZATION OF THE 0-NUMERICAL RANGE
Let H be a complex Hilbert space. Given a bounded linear operator A on H, we describe the set R n (A) = fV AW : V;W : C n ! H;V V = W W = In;V W = 0g. It is shown that the closed matricial convex hull of R n (A) is a closed ball of radius minfkA I k : 2 Cg centered at the origin.
On the kth matrix numerical range
Let A be an n × n complex matrix. The kth matrix numerical range of A is the set W(k:A) = {X * AX:X is an n×k matrix such that X*X=Ik }. We study the conditions on A under which the set W(k:A) is convex or starshaped. We get complete answers for hermitian matrices and partial results for general matrices. Then we consider different matrix sets S such as the collection of all scalar matrices, hermitian matrices, normal matrices, etc., and investigate the following problems: does AeS imply XeS for all XeW(k:A); what can we say about A if all XeW(k:A) are elements of S; if what is the largest k such that Some possible variations of the problem and related results are also discussed.
The Globalization problem for inner automorphisms and Skolem-Noether theorems
We investigate the circumstances under which an inner automorphism of a ring with local units can be built from “local” information. Specifically, we consider three natural “inner” type properties for an automorphism of a ring with local units. We show that every inner automorphism is locally inner but the converse is false, even if the automorphism is “piecewise” inner. On the other hand, we construct a large class of rings for which every locally inner automorphism is actually inner. Finally we obtain some Skolem-Noether type Theorems for infinite matrix and triangular matrix rings.