The set of numbers that contain no numbers. If there is no answer to some math problems, the answer is the null set.

Also called the empty set.

One formal method for defining the null set is:

ø = the set of all x such that x is not identical with x

This relies on the Axioms of Identity and Existence completely; the Axiom of Infinity is also important indirectly. It should be noted that, unlike other formal methods, this has no problem with multi-valued logic.

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