In mathematical logic, the diagonal lemma (also known as diagonalization lemma, self-reference lemma or fixed point theorem) establishes the existence of self-referential sentences in certain formal theories. A particular instance of the diagonal lemma was used by Kurt Gödel in 1931 to construct his proof of the incompleteness theorems as well as in 1933 by Tarski to prove his undefinability theorem.
In mathematical logic, the diagonal lemma (also known as diagonalization lemma, self-reference lemma or fixed point theorem) establishes the existence of self-referential sentences in certain formal theories. A particular instance of the diagonal lemma was used by Kurt Gödel in 1931 to construct his proof of the incompleteness theorems as well as in 1933 by Tarski to prove his undefinability theorem.
Source: Wikipedia "Diagonal lemma" · CC BY-SA 4.0
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