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Let $(X,d_{disc})$ be a discrete metric space, where d_{disc}(x,x')=\begin{cases} 1 & \text{if } x\neq x', \\ 0 & \text{if }x=x'.\end{cases}

Let $(Y,d_Y)$ be an arbitrary metric space, and $\mathbb{R}$ be equipped with d_{\mathbb{R}}(r,r')=|r-r'|.

  1. Prove that any function $f:X\to \mathbb{R}$ is continuous.
  2. Prove that $d_Y:Y\times Y\to \mathbb{R}$ is continuous. Here articulate which metric you use for $Y\times Y$.

My Idea:

  1. Observe that $f$ is continuous iff for all $\epsilon>0$, $\exists \delta>0$ such that d_{\mathbb{R}}(r,r')<\epsilon if d_{disc}(x,x')<\delta. Pick $\delta=1/2$ then if x=x' trivially d_{\mathbb{R}}(r,r')<\epsilon or else if x\neq x' then d_{\mathbb{R}}(r,r')=|r-r'|<\epsilon? I don't understand how to finish it. Alternatively, as Nate said I could look at the other definition, $f$ is continuous iff forall open sets $U\subset \mathbb{R}$ , $f^{-1}(U)\underset{open}{\subset} X.$ We are concerned with all the open balls with radius less than $1$.
  2. I am not sure how to proceed on this one. Any comments and hints will be appreciated.Thanks.
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    For your revised 1: if $x \ne x'$ then d(x,x') > \delta...2012-04-02

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