| 1. | The cocountable topology on a countable set is the discrete topology.
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| 2. | Essentially, yes, it means except for a countable set.
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| 3. | The distinction is made between a countable set and an uncountable set.
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| 4. | Some authors use countable set to mean " countably infinite " alone.
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| 5. | In mathematics, more precisely in measure theory, a at most countable set.
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| 6. | In discrete mathematics, countable sets ( including finite sets ) are the main focus.
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| 7. | If is a countable set for each in then the union of all is also countable.
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| 8. | For instance, the union of a countable set of countable sets need not be countable.
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| 9. | For instance, the union of a countable set of countable sets need not be countable.
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| 10. | ZF + AC ? suffices to prove that the union of countably many countable sets is countable.
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