 Metamath

Metamath is a formal language and an associated computer program (a proof checker) for archiving, verifying, and studying mathematical proofs.Several databases of proved theorems have been developed using Metamath covering standard results in logic, set theory, number theory, algebra, topology and analysis, among others.As of July 2019, the set of proved theorems using Metamath is one of the largest bodies of formalized mathematics, containing in particular proofs of 71 of the 100 theorems of the "Formalizing 100 Theorems" challenge, making it third after HOL Light and Isabelle, but before Coq, Mizar, ProofPower, Lean, Nqthm, ACL2, and Nuprl.