The problem is:

Let $X$ be a separable Banach space then there is an isometric embedding from $X$ to $\ell^{\infty}$.

My efforts:

I showed that there is an isometry from $X^*$ (topological dual) to $\ell^\infty$ in the following way:

Let $(e_{i})_{i=1}^{\infty}$ be a dense sequence in $B_{X}$ then define $\Phi:X^*\rightarrow\ell^\infty$ by $\Phi(f)=(f(e_{i}))_{i=1}^\infty$.

It is clear that $\Phi$ is an isometry.

An initial idea and a secondary question

Is there any canonical isometry from $X$ to $X^*$ since $X$ is separable (or not)?


2 Answers 2


Let $(x_n)_{n=0}^\infty$ be a dense sequence in $X$. For each $n$, find $x_n^*\in X^*$ so that $\|x^*_n\|=1$ and $x^*_n(x_n)=\|x_n\|$. It is easy to check that the operator $T: X\to \ell^\infty$ defined by $T(x)=(x_n^*(x))_{n=0}^\infty$ is the desired embedding.

  • 2
    $\begingroup$ Note that $|Tx|\leq|x|$ because $|x_{n}^*|=1$ for the other inequality let $x_{n_{j}}\rightarrow x$ then $|Tx_{n_{j}}|\geq |x_{n_{j}}|$ 'cause $x_{n_{j}}^*(x_{n_{j}})=|x_{n_{j}}|$ appears in the sequence $Tx_{n_{j}}$ taking the limit as $n_{j}$ goes to infinity we get the required inequality. $\endgroup$
    – checkmath
    Feb 24, 2012 at 1:48

This is almost the same as the other answers, with a bit of detail:

Let X be a separable Banach space. Let $(x_n)$ be a dense sequence in the unit sphere of $X$. For each $n$, use the Hahn-Banach Theorem to find a norm-one functional $f_n\in X^*$ with $f_n^*(x_n)=1$.

Define $\Phi: X\rightarrow\ell_\infty$ via $$ \Phi(x) =(f_n(x) ) $$

$\Phi$ is clearly linear.

Suppose $x\in X$ has norm one and let $1>\epsilon>0$. Choose $n_\epsilon$ so that $\Vert x_{n_\epsilon}-x\Vert<\epsilon$.

Then $$\epsilon>|f_{n_\epsilon}(x_{n_\epsilon}-x)|=|f_{n_\epsilon}(x_{n_\epsilon})-f_{n_\epsilon}(x)| = |1-f_{n_\epsilon}(x)|.$$

As $\epsilon$ was arbitrary, this implies that $\Vert \Phi(x)\Vert=\sup\limits_{n\in \Bbb N}|f_n(x ) |\ge 1$. Also, for any $n$, $$|f_n(x)|\le \Vert f_n \Vert \Vert x\Vert =1.$$ and so $\Vert \Phi(x)\Vert\le 1$.

Thus we have $\Vert \Phi(x)\Vert= 1$, whenever $\Vert x\Vert =1$.

From this it follows that for any non-zero element $x$ of $X$, we have
$$\Vert \Phi(x)\Vert= \Vert x\Vert \sup\limits_{n\in \Bbb N}|f_n(x/\Vert x\Vert) | =\Vert x\Vert;$$ and thus, $\Phi$ is an isometry.

  • $\begingroup$ Hi @David Mitra, I was looking your answer, and I don't understand two steps, I will be grateful if you can help me. First, how can we ensure that $f_{n}(x) \in l_{\infty}$? and second, I can't understand why if the norm of $x_{n}-x$ is less than $\epsilon$ why this implie that $f_{n}(x_{n}-x)$ is less than $\epsilon$? Thank You so much $\endgroup$
    – User43029
    Jun 8, 2013 at 18:46
  • $\begingroup$ @Charles 1) For every $n$, $|f_n(x)|\le \Vert f_n\Vert\Vert x\Vert\le 1\cdot\Vert x\Vert$. 2) Same argument as in $1)$ . $\endgroup$ Jun 8, 2013 at 19:11
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    $\begingroup$ Why do we need X be complete? It seems that the separability and Hahn-Banach do not require anything about completeness. $\endgroup$ Nov 20, 2019 at 21:33

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