Unordered Pair

From Encyclopediaofmath

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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