H. J. Keisler, J. H. Silver, "End Extensions of Models of Set Theory", p.177. In Axiomatic Set Theory, Part 1 (1971), Proceedings of Symposia in Pure Mathematics, Dana Scott, editor.
Kaufmann, Matt (1981), "On existence of Σn end extensions", Logic Year 1979–80, Lecture Notes in Mathematics, vol. 859, pp. 92–103, doi:10.1007/BFb0090942, ISBN3-540-10708-8
