# Unordered pair

From Encyclopedia of Mathematics

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

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

**How to Cite This Entry:**

Unordered pair.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Unordered_pair&oldid=54584