1. Adamowicz, Z. and Zbierski, P. (1997). Logic of Mathematics: A Modern Course of Classical Logic. Wiley, New York, NY.
2. Feferman, S. et al., editors (1986). Kurt Gödel Collected Works Volume I: Publications 1929–1936. Oxford University Press, Oxford.
3. Gödel, K. (1931). Über formal untentscheidbare Sätze der Principia mathematica und verwandter Systeme I. Monatshefte für Mathematik und Physik, 38:173–198. English translation by Jean van Heijenoort, “On formally undecidable propositions of Principia Mathematica and related systems I,” (Feferman et al., 1986, 144–195).
4. Kleene, S. C. (1986). Introduction to Gödel, 1931]. In [Feferman et al., 1986], pages 126–141.
5. Kreisel, G. (1957). A refinement of ω-consistency (abstract). The Journal of Symbolic Logic, 22:108–109.