Ernst Zermelo was a German mathematician known primarily for his foundational work in set theory. He was born on December 27, 1871, and died on May 21, 1953. Zermelo is most famous for developing the Zermelo-Fraenkel set theory (ZF), which is one of the most commonly used axiomatic set theories in mathematics.
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Ernst Zermelo in the 1900s
. Unknown photographer, Mondadori archive; Wikimedia Commons lists the photograph as public domain. Source.