One of the Zermelo-Fraenkel axioms which asserts the existence of the empty set 
. The axiom may be stated symbolically as
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Axiom of the Empty Set
See also
Zermelo-Fraenkel AxiomsExplore with Wolfram|Alpha
References
Itô, K. (Ed.). "Zermelo-Fraenkel Set Theory." §33B in Encyclopedic Dictionary of Mathematics, 2nd ed., Vol. 1. Cambridge, MA: MIT Press, pp. 146-148, 1986.Referenced on Wolfram|Alpha
Axiom of the Empty SetCite this as:
Weisstein, Eric W. "Axiom of the Empty Set." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/AxiomoftheEmptySet.html