Difference between revisions of "Countable"
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| − | A set X is countable if and only if there is a bijection from X to the set of [[Natural Numbers]]. Countable sets include the set of [[integer]]s and the set of [[rational number]]s. | + | A [[set]] X is '''countable''' if and only if there is a [[bijection]] from X to the set of [[Natural Numbers|natural numbers]]. Countable sets include the set of [[integer]]s and the set of [[rational number]]s. |
[[Category:Set theory]] | [[Category:Set theory]] | ||
Revision as of 17:40, October 15, 2007
A set X is countable if and only if there is a bijection from X to the set of natural numbers. Countable sets include the set of integers and the set of rational numbers.