1. Birkhoff, G., and von Neumann, J., Ann. of Math., 37, 823 (1936).
2. von Neumann, J., Lectures on Continuous Geometries, theorems 5.3 and 5.4, vol. 1 (Princeton Univ. Press, 1936–7). See also Birkhoff, G., Lattice Theory, second ed., 171 (American Mathematical Society, 1948), and Dilworth, R. P., Bull. Amer. Math. Soc., 46, 74 (1940).
3. Birkhoff, G., and Ward, M., Ann. of Math., 40, 609 (1939). See also Birkhoff, G., Lattice Theory, second ed., 170 (American Mathematical Society, 1948).
4. Birkhoff, G., Lattice Theory, first ed., 94 (American Mathematical Society, 1940); second ed., 171 (American Mathematical Society, 1948).
5. Tarski, A., Logic, Semantics, Metamathematics, Theorem 11, 45 (Oxford Univ. Press, 1956).