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Let $f$ be a real-valued function defined on $\mathbb{R}$. Show that the set of points at which $f$ is continuous is a countable intersection of open sets.

Not sure where to start on this one... what would be the open sets?

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marked as duplicate by Zev Chonoles, user67258, Amzoti, Brian Rushton, PJ Miller Jun 28 '13 at 2:58

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