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Prove that the intersection of infinite number of open sets is not necessarily open.

Formal proof required

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$$\bigcap_{n \geq 1} \left(-\frac 1n, \frac 1n\right)$$

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    How is this a "formal proof"? – 2017-02-18
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    @Xam Is this site supposed to be a "do my homework for free even when I show no research effort" site? This answer seems perfectly fine to me. The OP should work out the details by himself from this hint. Also a "formal" proof would be in some formal system!! – 2017-02-18
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    @NeedForHelp actually, given the way the OP asked his/her question, no one should have answered it. – 2017-02-18
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    @Xam That's funny, because many questions you've answered aren't asked much better. – 2017-02-18
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    @MathematicsStudent1122 many questions? are you sure? btw, I just want to point out that these questions should not be answered, but if for some reason someone decide to answer it, they should be at least clear enough to help the OP. I doubt that in this case the OP would understand your "hint". – 2017-02-18