I'm trying to understand Borel sets. I am looking for a visual (i.e., constructive) $F_{\sigma\delta}$ subset of $[0,1]$ of measure $1$ which is not $F_\sigma$. Any idea or suggestion would be appreciated.
2026-03-28 09:53:31.1774691611
An example of a $F_{\sigma\delta}$ subset of $[0,1]$ of measure $1$ which is not $F_\sigma$
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Use $[0,1]\setminus\Bbb Q$: it’s clearly a $G_\delta$ and therefore an $F_{\sigma\delta}$, but it’s not an $F_\sigma$.