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?


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

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