In 1922, Adolf Fraenkel and Thoralf Skolem independently improved Zermelo's axiom system. The resulting 10 axiom system, now called Zermelo-Fraenkel axioms (ZF), is now the most commonly used system for axiomatic set theory.
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Anonymous2013-09-01 15:50
Some basic sets of central importance are the empty set (the unique set containing no elements), the set of natural numbers, and the set of real numbers.