Editing Naive set theory (section)

=== Empty set ===
The [[empty set]], denoted as <math>\varnothing</math> and sometimes <math>\{\}</math>, 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]].){{sfn|Halmos|1974|p=9}} Although the empty set has no members, it can be a member of other sets. Thus <math>\varnothing\neq\{\varnothing\}</math>, because the former has no members and the latter has one member.{{sfn|Halmos|1974|p=10}}