Frege and Gödel - Two Fundamental Texts in Mathematical LogicJean van Heijenoort (ed.)Begriffsschrift, a formula language, modeled upon that of arithmetic, for pure thought (1879), by G. Frege
Some metamathematical results on completeness and consistency; On formally undecidable propositions of Principia mathematica and related systems I; and On completeness and consistency (1930b, 1931, and 1931a), by K. Gödel
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