Definition:Doubleton/Class Theory

Definition
A doubleton is a class:


 * $\left\{{a, b}\right\} = \left\{{x: x = a \lor x = b: a \ne b}\right\}$

Also see

 * Axiom:Axiom of Pairing


 * Union of Disjoint Singletons is Doubleton for a proof from the ZF axioms that $\left\{{a}\right\} \cup \left\{{b}\right\} = \left\{{a, b}\right\}$ when $a \ne b$.