1. N. I. Akhiezer and I. M. Glazman, Theory of linear operators in Hilbert space, Translated from the Russian and with a preface by Merlynd Nestell. Reprint of the 1961 and 1963 translations. Two volumes bound as one. Dover Publications, Inc., New York, 1993.
2. A. Ben-Israel and T. N. E. Greville, Generalized inverses: theory and applications, Wiley-Interscience, New York, 1974.
3. M. S. Birman and M. Z. Solomjak, Spectral theory of selfadjoint operators in Hilbert space (Russian), Leningrad. Univ., Leningrad, 1980.
4. Carvajal, Xavier; Neves, Wladimir. Operators that achieve the norm, Integral Equations Operator Theory, 72(2) (2012), 179–195.
5. X. Carvajal and W. Neves, Operators that attain their minima, Bull. Braz. Math. Soc. (N.S.), 45(2) (2014), 293–312.