↑ Halbeisen, Lorenz J. (2011), Combinatorial Set Theory: With a Gentle Introduction to Forcing , Springer Monographs in Mathematics, Springer, pp. 380–381 , ISBN9781447121732。
↑ Soare, Robert I. (1987), Recursively Enumerable Sets and Degrees: A Study of Computable Functions and Computably Generated Sets , Perspectives in Mathematical Logic, Springer, p. 245, ISBN9783540152996。