1. Sakai, S.:C*-algebras andW*-algebras. Berlin-Heidelberg-New York: Springer 1971
2. Sinai, Y., Helemski, A.: Funk. An-i Prit.6, 99 (1972); Translated in Func. An. Appl.6, 343 (1973)
3. Sakai, S.: University of Pennsylvania Preprint (1974)
4. Hille, E., Phillips, R.S.: Functional analysis and semi-groups. AMS, Providence (1957)
5. Nelson, R.: Ann. Math.70, 572 (1959)