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  "body": "\n## Definition\n\nA [[set|0SJ3RX9TXA]] $A$ is countably infinite when there is a [[bijective function|0SNQEF7Z53]] $f \\colon \\mathbb{N} \\to A$.\n\n## Remarks\n\nAnother name is denumerable. Its cardinality is [[aleph-null|0V2H5PGJ9B]].\nExamples are $\\mathbb{N}$, $\\mathbb{Z}$ and $\\mathbb{Q}$, and also the set of primes.\nA set is countably infinite exactly when it is [[countable|0ZG61TFBES]] and [[infinite|01MKW4R283]].\n\n## Explanations\n\n### L1 human\n\nA countably infinite set can be listed as $a_0, a_1, a_2, \\dots$ without end, and each element appears exactly one time.\nThe integers have such a list: $0, 1, -1, 2, -2, \\dots$.\n",
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