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=                             Empty set                              =
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                             Introduction
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In mathematics, the empty set is the unique set having no elements;
its size or cardinality (count of elements in a set) is zero. Some
axiomatic set theories ensure that the empty set exists by including
an axiom of empty set; in other theories, its existence can be
deduced. Many possible properties of sets are vacuously true for the
empty set.

In some textbooks and popularizations, the empty set is referred to as
the "null set".  However 'null set' is a distinct notion within the
context of measure theory. In that setting, it describes a set of
measure zero; such a set is not necessarily empty.  The empty set may
also be called the 'void set'.  Common notations for the empty set