1. T. Furuta, J. Mićić, J. Pečarić and Y. Seo, Mond-Pečarić Method in Operator Inequalities. Inequalities for Bounded Selfadjoint Operators on a Hilbert Space, Element, Zagreb, 2005.
2. G. Helmberg, Introduction to Spectral Theory in Hilbert Space, John Wiley & Sons, Inc., New York, 1969.