International Journal of Mathematics and Mathematical Sciences
Volume 2008 (2008), Article ID 362409, 6 pages
doi:10.1155/2008/362409
Research Article
Some Estimates of Certain Subnormal and Hyponormal Derivations
Department of Mathematics, Florida A&M University, Tallahassee, FL 32307, USA
Received 23 October 2007; Accepted 9 January 2008
Academic Editor: Nils Ackermann
Copyright © 2008 Vasile Lauric. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
We prove that if A and B∗ are subnormal operators and X is a bounded linear operator such that AX−XB is a Hilbert-Schmidt operator, then f(A)X−Xf(B) is also a Hilbert-Schmidt operator and
‖f(A)X−Xf(B)‖2≤L‖AX−XB‖2
for f belongs to a certain class of functions. Furthermore, we investigate the similar
problem in the case that S, T are hyponormal operators and X∈ℒ(ℋ) is such that SX−XT belongs to a norm ideal (J,‖⋅‖J), and we prove that f(S)X−Xf(T)∈J and
‖f(S)X−Xf(T)‖J≤C‖SX−XT‖J
for f being in a certain class of functions.