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I came across the following problem on open sets:

Let $\mathcal{C}$ be a set of nonempty open sets in $\mathbb{R}$ such that for $U,V \in \mathcal{C}$, either $U=V$ or $U \cap V = \emptyset$. Prove that either $\mathcal{C}$ is finite or the sets in $\mathcal{C}$ can be listed in a sequence $(U_n)$.

Now we can enumerate the rational numbers using Farey sequences. So choose a rational number from each $U \in \mathcal{C}$. Form a neighborhood about each rational number from the open sets and see if they (the neighborhoods) intersect or not.

So it suffices that if we choose a rational number from each open set, by countability of the rationals $\mathcal{C}$ is countable?

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    What is your question? Seems unclear to me.2011-06-27
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    Damien: a little piece of advice. The number of questions you can ask here is limited by the software to 50 a month, [see here](http://meta.math.stackexchange.com/questions/2302/do-we-limit-how-often-someone-may-ask-about-their-homework/2303#2303). You asked 26 questions in about a week, so maybe you want to slow down a little.2011-06-27
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    The "Form a neighborhood $\dots$ or not." sentence is superfluous. The way one enumerates the rationals doesn't make any difference to the argument. But Farey sequences don't enumerate the rationals.2011-06-27

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