General Topology – Which Metric Spaces are Separable?

functional-analysisgeneral-topologymetric-spacesseparable-spaces

which of the following metric spaces are separable?

  1. $C[0,1]$ with usual 'sup norm' metric.

  2. the space $l_1$ of all absolutely convergent real sequences, with the metric $$d_1(a_i,b_i)=\sum_{1}^{\infty}|a_i-b_i|$$

  3. The space $l_{\infty}$ of all bounded real sequences with the metric
    $$d_{\infty}(a_i,b_i)=\sup|a_i-b_i|$$

Well, 1 is separable as polynomials are dense in $C[0,1]$ so I can construct a set of polynomial with rational coefficients that is going to be a countable dense set for $C[0,1]$

I have no idea about 2,3 .

Well, along with this question I just want to ask The closed unit ball is compact with respect to $l_1$ metric? I guess no, because Sequence $e_1=(1,0,\dots),\dots e_n=(0,0,\dots,1(nth place),0,0\dots)$ this seqquence has no convergent subsequence so not sequentially compact. Am I right?

Best Answer

Another possibility for (3): You have $F=\{0,1\}^{\mathbb{N}} \subset \ell_{\infty}$ and moreover for all $x,y \in F$, if $x \neq y$ then $d(x,y) =1$. So $\{B(x,1),x \in F\}$ is a family of disjoint open sets whereas $F$ is uncountable.

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