In a metric space let $A$ be compact and $B$ - open, dense everywhere. Does it mean that $ \overline{A} = \overline{A\cap B} $ or there are counterexamples?
Intersection of open and compact
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real-analysis
general-topology
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0Thanks for the post; it reminds me to not jump to the conclusion that $ \overline{A\cap B}$=$\overline{A}\cap\overline{B}$, and examples like A$=(0,1)$ and B=$(1,2)$ – 2011-06-30
1 Answers
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What about $A=\{0\}$ and $B=\mathbb R\setminus\{0\}$?
I assume you use everywhere dense as a synonym to dense - like here.
Perhaps this result is worth mentioning in this context: If $A$ is dense in $X$, then for every open $U\subset X$ we have $\overline U=\overline{U\cap A}$.