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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PJ Miller
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Also http://math.stackexchange.com/q/67620/264 and http://math.stackexchange.com/q/211511/264 – Zev Chonoles Jun 28 '13 at 01:31
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Where to start? How about the definition of continuity? – Ted Shifrin Jun 28 '13 at 01:42