4
$\begingroup$

Consider the normed spaces (over the field of real numbers) $X=(\ell_\infty,\|\cdot\|_\infty)$ and $Y=(\ell_\infty,\|\cdot\|)$ where $\|x\|=\sup_{n\in\mathbf{N}}\frac{|x_n|}{2^n}.$

How can I show that the closed unit ball in $X$ is compact in $Y$?

  • 0
    Hum. How can the unit ball be precompact in a infinite-dimensional normed space? I guess that $Y$ is just a metric space, not a normed space.2012-09-07

1 Answers 1

1

We show that $B$, the unit ball in $X$, is precompact and complete.

  • Precompactness: we fix $\varepsilon>0$ and $N$ such that $\frac 1{2^N}\leq \varepsilon$. Using the fact that $[-1,1]^N$ is precompact, we get $v_1,\dots,v_k\in [-1,1]^N$ such that $[-1,1]^N\subset \bigcup_{j=1}^kB(v_j,\varepsilon)$, where $B$ is the ball in $\Bbb R^N$ endowed with the supremum norm. Then take $w_j:=(v_j,0,\dots,)$.

  • Completeness: consider $\{x^{(n)}\}\subset B$ a Cauchy sequence for $\lVert\cdot\rVert$. For each fixed $k$, $\{x^{(n)}_k\}_n$ is a Cauchy sequence in $\Bbb R$, hence converges to some $x_k$. For a fixed $\varepsilon>0$, we take $N$ such that $\frac 1{2^N}\leq \varepsilon$ and since all the terms of the sequence are bounded by $1$ we just work with a finite number of entries.