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 Danish and Norwegian alphabet (and not related in any way to the Greek letter Φ). Other notations for the empty set include "Λ" and "0".
The empty-set symbol ∅ is found at Unicode point U+2205. In TeX, it is coded as \emptyset or \varnothing.
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