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Suppose $(X,d)$ is a metric space where $A\subseteq X$ is totally covered i.e. $\forall \epsilon >0$ $\exists \{a_1,a_2,\ldots,a_m\}\in A $ s.t $A\subseteq \bigcup_{1,\ldots,m}B(a_i,\epsilon)$. Given that $B\subseteq A$ show that $B$ is totally covered.

I can't seem to show - with the given definition - that the centers of the balls would now come from $B.$

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2 Answers 2

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So $B\subseteq\bigcup_{i=1}^{m}B_{\epsilon/2}(a_{i})$ by choosing $\epsilon/2$-cover for $A$, and assume without loss of generality that $B_{\epsilon/2}(a_{i})\cap B\ne\emptyset$ for all $i=1,...,m$. Pick a $b_{i}\in B_{\epsilon/2}(a_{i})\cap B$. We claim that $\{B_{\epsilon}(b_{i})\}_{i=1}^{m}$ cover $B$:

For $b\in B$, pick an $i$ such that $b\in B_{\epsilon/2}(a_{i})$. Then $d(b,b_{i})\leq d(b,a_{i})+d(a_{i},b_{i})<\epsilon$, so $b\in B_{\epsilon}(b_{i})$, as expected.

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    $\begingroup$ Just a suggestion: had you first taken a cover of $\epsilon/2$ balls of $A$, and from that constructed a cover of $\epsilon$ balls of $B$, that would have made the answer even clearer as per the question (which was given any $\epsilon$, find a finite cover of $\epsilon$ balls of $B$). $\endgroup$
    – user67803
    Feb 26, 2018 at 3:46
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Suppose $B \subseteq A$ and let $\varepsilon > 0$. Find a finite covering of $A$ by balls of radius $\frac{\varepsilon}{2}$, say $$B_{\varepsilon/2}(a_1),\ \dots,\ B_{\varepsilon/2}(a_n)$$ where $a_1,\dots,a_n \in A$.

By relabelling the elements $a_1,\dots,a_n$ if necessary, you can assume that the first $k$ of these balls contains an element of $B$, and the remaining $n-k$ do not. For each $1 \le i \le k$, choose some $b_i \in B_{\varepsilon/2}(a_i)$, and note that $B_{\varepsilon}(b_1),\dots,B_{\varepsilon}(b_k)$ is a cover of $B$.

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