1. J. W. Addison,The theory of hierarchies, in Logic, Methodology, and Philosophy of Science, Eds. Nagel, Suppes, and Tarski, Stanford, 1962, 26–37.
2. C. C. Chang,Ultraproducts and other methods of constructing models, to appear in Sets, Models mand Recursion Theory. ed. John Crossley, North Holland.
3. G. Fuhrken,Languages with the quantifier “there exist at least X α”. in The Theory of Models, eds. J. Addison, L. Henkin, and A. Tarski, Amsterdam, 1965, pp. 121–131.
4. B. Jónsson,Homogeneous universal relational systems, Math. Scand.8 (1960), 137–142.
5. H. J. Keisler,Homogeneous theories (abstract), Notices Amer. Math. Soc.12 (1965), 600.