# Open cover of a metric space

I am trying to find a definition for the open cover of a metric space, but i cannot find it. So, if X is a metric space and A is a subset of X, then what is the definition for open cover of A? Can anyone help? Thank you.

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Do you know what an open set (in a metric space) is? – Adam Saltz Dec 4 '12 at 19:59

## 2 Answers

An open cover of $A$ in $X$ is simply a family $\mathscr{U}$ of open sets in $X$ such that $A\subseteq\bigcup\mathscr{U}$. A relatively open cover of $A$ as a subspace of $X$ is a family $\mathscr{U}$ of open sets in $A$, i.e., of sets of the form $U\cap A$ for some open $U$ in $X$.

Example: Let $X=\Bbb R$, and let $A=(0,1)$. Then $$\mathscr{U}=\left\{\left(\frac1n,1\right):n\in\Bbb Z^+\right\}$$ is an open cover of $A$, because each $x\in A$ belongs to at least one member of $\mathscr{U}$. Specifically, if $x\in A$, then $x>0$, so there is a positive integer $n$ such that $\frac1n<x$, and $x\in\left(\frac1n,1\right)\in\mathscr{U}$.

Finally, a set $U\subseteq X$ is open if and only if it is a union of open balls: for each $x\in U$ there is an $\epsilon_x>0$ such that $B(x,\epsilon_x)\subseteq U$, where $B(x,\epsilon_x)=\{y\in X:d(x,y)<\epsilon_x\}$.

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Thanks, when you say "family of" open sets, do you mean union of open sets? – user49065 Dec 4 '12 at 20:02
@user49065: No, family, collection, and set all mean the same thing: $\mathscr{U}$ is a set of open sets in $X$. I used family only because a set of sets sounds awkward. – Brian M. Scott Dec 4 '12 at 20:06
@user49065: I’ve added a concrete example that may help make it clearer. – Brian M. Scott Dec 4 '12 at 20:14
Hi Brian. Did you mean to say $A\subseteq \cup \mathscr{U}$ in the first line? And $X$ instead of $M$? – Thomas E. Dec 5 '12 at 12:36
@ThomasE.: Yes, and yes; and thanks for catching them. – Brian M. Scott Dec 5 '12 at 21:19

An open cover of a subset $A \subseteq X$, is a collection $\{U_i\}_{i \in I}$ of open sets in $X$, such that

$$A \subseteq \bigcup_{i \in I} U_i$$

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