1. K. R. Apt, W. Marek,Second order arithmetic and related topics,Annals of Mathematical Logic, Vol. 6 (1974), pp. 177–229.
2. K. J. Barwise,Admissible sets over models of set theory, inGeneralized recursion theory, ed. by J. E. Fenstad, P. G. Hinman, North Holland Publ. Comp. 1974, pp. 97–122.
3. K. J. Barwise, J. Schlipf,On recursively saturated models of arithmetic (mimeographed).
4. C. C. Chang, H. J. Keisler,Model theory, North Holland Publ. Comp. 1973.
5. A. Ehrenfeucht, G. Kreisel,Strong models of arithmetic,Bulletin de l'Académie Polonaise des Sciences,Serie des sci. math., astr. et phys., Vol. 14 (1966), pp. 107–110.