1. J. Barwise, Admissible Sets and Structures, Berlin (1975).
2. Yu. L. Ershov, Problems of Decidability and Constructive Models [in Russian], Nauka, Moscow (1980).
3. C. C. Chang and H. J. Keisler, Model Theory, North-Holland, Amsterdam (1973).
4. M. Makkai, "Admissible sets and infinite logic," in: Reference Book on Mathematical Logic [in Russian], Vol. 1, Nauka, Moscow (1982).