1. Akhiezer, N.I., Glazman, I.M.: Theory of linear operators in Hilbert space. Translated from the third Russian edition by E. R. Dawson. Translation edited by W. N. Everitt. Monographs and Studies in Mathematics, 10, vol. II. Pitman (Advanced Publishing Program), Boston (1981)
2. Alonso, A., Simon, B.: The Birman–Kreĭn–Vishik theory of self-adjoint extensions of semibounded operators. J. Oper. Theory 4, 251–270 (1980)
3. Anderson, W.N., Duffin, B.J.: Series and parallel addition of matrices. J. Math. Anal. Appl. 26, 576–594 (1969)
4. Anderson, W.N., Trapp, G.E.: Shorted operators. II. SIAM J. Appl. Math. 28, 60–71 (1975)
5. Ando, T., Nishio, K.: Positive self-adjoint extensions of positive symmetric operators. Tohóku Math. J. 22, 65–75 (1970)