1. American Mathematical Society Colloquium Publications, Vol. 25;Birkhoff, Garrett,1948
2. On closed elements in closure algebras;McKinsey, J. C. C.;Ann. of Math. (2),1946
3. Pseudo-complements in semi-lattices;Frink, Orrin;Duke Math. J.,1962
4. V. Glivenko, Sur quelques points de la logique de M. Brouwer, Bull. Acad. Sci. Belg. 15 (1929), 183-188.