1. American Mathematical Society Colloquium Publications, Vol. 25;Birkhoff, Garrett,1948
2. C. C. Chang, Bull. Amer. Math. Soc. Abstract 61-4-579 (1955).
3. Boolean algebras and fields of sets;Enomoto, Shizu;Osaka Math. J.,1953
4. On families of mutually exclusive sets;Erdös, P.;Ann. of Math. (2),1943
5. Measures in Boolean algebras;Horn, Alfred;Trans. Amer. Math. Soc.,1948