1871-1953

German mathematician best known for his work in set theory. He was the first to prove the well-ordering theorem (1904), a result important for many applications of set theory. Widely challenged, Zermelo responded with a second proof of the theorem in 1908. The same year he produced the first axiomatic formulation of set theory. Zermelo later placed certain restrictions on set formation to avoid paradoxes that otherwise would have rendered his axiomatic system useless.