Editing Empty set (section)

=== Historical issues ===
In the context of sets of real numbers, Cantor used <math>P\equiv O</math> to denote "<math>P</math> contains no single point". This <math>\equiv O</math> notation was utilized in definitions; for example, Cantor defined two sets as being disjoint if their intersection has an absence of points; however, it is debatable whether Cantor viewed <math>O</math> as an existent set on its own, or if Cantor merely used <math>\equiv O</math> as an emptiness predicate. Zermelo accepted <math>O</math> itself as a set, but considered it an "improper set".<ref>A. Kanamori, "[https://math.bu.edu/people/aki/8.pdf The Empty Set, the Singleton, and the Ordered Pair]", p.275. Bulletin of Symbolic Logic vol. 9, no. 3, (2003). Accessed 21 August 2023.</ref>