Metamathematics Cut-elimination Tarski's undefinability theorem Diagonal lemma Provability logic Interpretability logic Sequent Sequent calculus Analytic proof Structural proof theory Self-verifying theories Substructural logics Structural rule Weakening Contraction Linear logic Intuitionistic linear logic Proof net Affine logic Strict logic Relevant logic Proof-theoretic semantics Ludics System F Gerhard Gentzen Gentzen's consistency proof Reverse mathematics Nonfirstorderizability Interpretability Weak interpretability Cointerpretability Tolerant sequence Cotolerant sequence Deduction theorem Cirquent calculus