1. C. W. Helstrom, Quantum Detection and Estimation Theory, Academic Press, (New York, 1976). The reader is also addressed to the original papers:
2. C. W. Helstrom, Found. Phys. 4, 453 (1974);
3. C. W. Helstrom, Int. J. Theor. Phys. 11, 357 (1974).
4. M. A. Naimark, Iza. Akad. Nauk USSR, Ser. Mat. 4 277 (1940).
5. N. I. Akhierzer and I. M. Glezman, Theory of Linear Operators in the Hilbert Space, (two volumes bound as one), Dover Publ. Inc. N.Y 1993, pag. 121 of Vol. 2.