If $A$ and $A \cap B$ are in a $\sigma$-algebra, is $B$ in the $\sigma$-algebra? I thought this was true, but I seem to be unable to prove it.
2026-05-03 15:14:26.1777821266
If $A$ and $A \cap B$ are in a $\sigma$-algebra, is $B$ in the $\sigma$-algebra?
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