1. Dunford N. and Schwartz J.T., Linear Operators. Part 2: Spectral Theory. Interscience Publishers, New York/London (1963).
2. Mikhlin S.G., The Problem of the Minimum of a Quadratic Functional. Gostekhizdat, Moscow – Leningrad, (1959). (in Russian) [English transl. Holden Day, San Francisco, 1965].