#set-theory

Set theory is the mathematical theory of collections of objects that are called elements of the set

https://en.wikipedia.org/wiki/Set_theory http://www-history.mcs.st-and.ac.uk/HistTopics/Beginnings_of_set_theory.html http://www.settheory.net/ http://plato.stanford.edu/entries/set-theory/ https://en.wikibooks.org/wiki/Set_Theory