Unordered pair

2020 Mathematics Subject Classification: Primary: 03E [MSN][ZBL]

doubleton

A set containing two elements, denoted $\\{x,y\\}$. One has $z \in \\{x,y\\} \Leftrightarrow z = x \vee z = y$, and so $\\{x,y\\} = \\{y,x\\}$. Compare with an ordered pair.

#### References[edit]

[1] |  P. R. Halmos, Naive Set Theory, Springer (1960) ISBN 0-387-90092-6

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