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Prove that if A is a set on [0,1] and An is its $(1/n)$ neighborhood, i.e. $A_n= \{x \in R: \exists y \in A, |x-y|< 1/n \}$ . then $\cap_{n=1}^{\infty} A_n= \overline{A}$, where $\overline{A}$ is the closure of A.

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    Hint: A point $x$ is *outside* $\overline{A}$ if there is an open ball around $x$ that is disjoint from $A$. For any such $x$, there will be an $n$ such that $x \notin A_n$.2012-08-05
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    what have you done so far? Do you see why any point not in the closure is going to be excluded by some sufficiently high $n$?2012-08-05
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    Don't give orders please, instead *ask* for help and show us what you've already tried. Start by writing down the required definitions (in this case of $\bar A$).2012-08-05
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    I am just trying to prove it throught definition. I consider that $A_1 \supset A_2 \supset .... \supset A_n$, so that $A_n$ has the same feature with the definition of closure. But I can not work out a clear proof.2012-08-05
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    I will try to do the double direction thing.2012-08-05

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