1. Boolos, G.: 1978, The Unprovability of Consistency: An Essay in Modal Logic (Cambridge University Press, Cambridge).
2. Boolos, G. and R. Jeffrey: 1974, Computability and Logic (Cambridge University Press, Cambridge).
3. Detlefson, M.: 1979, ?On interpreting Gödel's Second Incompleteness Theorem?, Journal of Philosophical Logic 8, pp. 297?315.
4. Feferman, S.: 1960, ?Arithmetization of Metamathematics in a General Setting?, Fundamenta Mathematicae XLIX, pp. 35?92.
5. Feferman, S.: 1962, ?Transfinite Recursive Progressions of Axiomatic Theories?, JSL XXVII, pp. 259?316.