(n,l)-sums of models

H. J. Keisler, Unions of Relational Systems, Proc. Amer. Math. Soc.15, No. 4, 540–545 (1964). H. J. Keisler, Theory of Models with Generalized Atomic Formulas, J. Symbolic Logic,25, 1–26 (1960). A. D. Taimanov, Characteristics of Axiomatizable Classes of Models, I, Izv. Akad. Nauk SSSR, Ser. Matem.,25, No. 4, 601–620 (1961). A. D. Taimanov, Characteristics of Axiomatizable Classes of Models, II, Izv. Akad. Nauk SSSR, Ser. Matem.,25, No. 6, 755–765 (1961). A. D. Taimanov, Characteristics of Axiomatizable Classes of Models, In: Algebra and Logic [in Russian], I, Vol. 4, Novosibirsk (1962), pp. 5–31. A. Tarski, Contributions to the Theory of Models, I, II, Indag. Math.16, 572–581; 582–588 (1954). R. Vaught, Remarks on Universal Classes of Relational Systems, Indag. Math.16, 589–591 (1954). J. Los, On the Extending of Models, I, Fund. Math.,42, 38–54 (1955). J. Los and R. Suszko, On the Extending of Models, IV, Fund. Math.,44, 52–60 (1957). C. C. Chang, On Unions of Chains of Model, Proc. Amer. Math. Soc.,10, 120–127 (1959). L. Henkin, Two Concepts from the Theory of Models, J. Symbolic Logic,21, 28–32 (1956). A. I. Malatsev, Some Questions in the Modern Theory of Classes of Models, In: Proceedings of the Fourth All-Union Mathematics Conference [in Russian], Vol. 1, Leningrad (1963), pp. 169–195. Yu. L. Ershov, On Axiomatizable Classes of Models with Infinite Signatures, In: Algebra and Logic [in Russian], Im Vol. 4, Novosibirsk (1962), pp. 32–44. D. A. Zakharov, Concerning the Les-Suzzko Theorem, Uspekhi Matem. Nauk,16, No. 2, 200–201 (1961).