1. Conditional expectation in an operator algebra;Umegaki;Tôhoku Math. J.,1954
2. Expectations in an operator algebra;Nakamura;Tôhoku Math. J.,1954
3. Formes linéaires sur un anneau d'opérateurs;Dixmier;Bull. Soc. Math. France,1953
4. An operational approach to quantum probability;Davies;Comm. Math. Phys.,1970
5. Positive definite measures;Blattner,1963