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A doubleton is a set that contains exactly two elements.

The doubleton containing the distinct elements $a$ and $b$ can be written $\set {a, b}$.

The set $\set {a, b}$ is known as the doubleton of $a$ and $b$.

Class Theory Definition

Let $a$ and $b$ be sets.

The class $\set {a, b}$ is a doubleton (class).

It is defined as the class of all $x$ such that $x = a$ or $x = b$:

$\set {a, b} = \set {x: x = a \lor x = b: a \ne b}$

Also known as

A doubleton is also known as an unordered pair, or even simply pair set or pair if no ambiguity results.

Also see

  • Results about doubletons can be found here.