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In mathematics, the null sign ((unicode:∅)) denotes the empty set. 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. )〕 In linguistics, the null sign is used to indicate the absence of an element, such as a sound or morpheme. It is also used in distinguishing the symbol null from zero (0). ==History of Null Sign== Sets become a mathematical branch in the early years.An argument was held by the mathematicians on how to distinguish a set that has no elements in it.They suggested the letter "O",but mathematicians disagreed because O can be an element.Zero is also a choice but the same thing with O,it can be an element.So the group invented the slashed zero or slashed O.(∅) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「null sign」の詳細全文を読む スポンサード リンク
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