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Show that every neighborhood is an open set.

Let $E = N_{r}(p)$. Then I want to show that for every $q \in E$, $q$ is an interior point. This means I just have to find a neighborhood of $q$ which is in $E$?

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    How do *you* define *neighborhood*?2011-08-22

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Dylan's suggestion of drawing a picture is good. Here we have our point $p$, the open ball $E=N_r(p)$, and an arbitrary point $q\in E$. I have labeled the distance from $p$ to $q$ as $d$.

enter image description here

If Dylan and I understand the question correctly, you want to show that there is some $s>0$ for which the open ball $N_s(q)$ is contained in $E$ (i.e. $N_s(q)\subseteq E$)? If that's the case, then can you think of an $s$, depending on $r$ and $d$, for which this is true? Think about drawing the circle of radius $s$ around the point $q$ in the picture.

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    @James: Yup! This follows from the [triangle inequality](http://en.wikipedia.org/wiki/Metric_space#Definition), which is part of the axioms of a metric space.2011-08-23