information:extraction, structural:expansion
Empty set

The empty set, denoted as ∅ {\displaystyle \varnothing } and sometimes { } {\displaystyle \{\}}, is a set with no members at all. Because a set is determined completely by its elements, there can be only one empty set. (See axiom of empty set.) Although the empty set has no members, it can be a member of other sets. Thus ∅ ≠ { ∅ } {\displaystyle \varnothing \neq \{\varnothing \}}, because the former has no members and the latter has one member.