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  "body": "\n## Summary\n\nAn $F_\\sigma$ set in a [[topological space|0CQHFPGJ45]] is a union of countably many [[closed sets|0ZCK8DQYN1]].\nThe name comes from the French words for closed set and sum.\nThe complements of $F_\\sigma$ sets are the [[G-delta sets|V1TBNP8R3S]], and in a [[metric space|0VKD1TP7F3]] every open set is $F_\\sigma$.\n\n## Explanations\n\n### L1 human\n\nA single point is closed, so any countable set is a countable union of closed sets.\nFor example, the set $\\Q$ of rational numbers is $F_\\sigma$ in $\\R$.\nAn open interval is also $F_\\sigma$, since $(0, 1)$ is the union of the closed intervals $[1/n, 1 - 1/n]$.\nThe irrational numbers, however, are not $F_\\sigma$ in $\\R$.\n",
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