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Empty set
In mathematics, and more specifically set theory, 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 trivially true for the empty set.
''Null set'' was once a common synonym for "empty set", but is now a technical term in measure theory. The empty set may also be called the ''void set''.
==Notation==
Common notations for the empty set include "{}", "∅", and "". The latter two symbols were introduced by the Bourbaki group (specifically André Weil) in 1939, inspired by the letter Ø in the Norwegian and Danish alphabets (and not related in any way to the Greek letter Φ).〔(Earliest Uses of Symbols of Set Theory and Logic. )〕
The empty-set symbol (unicode:∅) is found at Unicode point U+2205.〔(Unicode Standard 5.2 )〕 In TeX, it is coded as \emptyset or \varnothing.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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