A diagonal argument, in mathematics, is a technique employed in proofs. The following theorems are notable examples: Cantor's diagonal argument (the earliest) Cantor's theorem Russell's paradox Curry's paradox Diagonal lemma Gödel's first incompleteness theorem Tarski's undefinability theorem Halting problem Kleene's recursion theorem Lawvere's fixed-point theorem (categorical generalization of all of the above)
See also Diagonalization (disambiguation)