Abstract
We show that frames with frame bounds A and B are images of orthonormal bases under positive operators with spectrum contained in . Then, we give an explicit characterization of the diagonals of such operators, which in turn gives a characterization of the sequences which are the norms of a frame. Our result extends the tight case result of Kadison [Proc. Natl. Acad. Sci. USA 99: 4178–4184, 2002], [Proc. Natl. Acad. Sci. USA 99: 5217–5222, 2002], which characterizes diagonals of orthogonal projections, to a non-tight case. We illustrate our main theorem by studying the set of possible lower bounds of positive operators with prescribed diagonal.
© Walter de Gruyter Berlin · New York 2011