Ullrich, Dirk. “A General Principle for Purely Model-Theoretical Proofs of Gödel’s Second Incompleteness Theorem”. Logic and Logical Philosophy 6, no. 6 (November 6, 1998): 173–185. Accessed July 5, 2024. https://apcz.umk.pl/LLP/article/view/LLP.1998.009.