User:Caliburn/s/fa/Normed Vector Space with Separable Dual is Separable
Theorem
Let $\struct {X, \norm \cdot_X}$ be a normed vector space.
Let $\struct {X^\ast, \norm \cdot_{X^\ast} }$ be the normed dual space of $X$.
Then, if $X^\ast$ is separable:
- $X$ is separable.
Proof
From Characterization of Separable Normed Vector Space, we have that:
- $S_{X^\ast} = \set {f \in X^\ast : \norm f_{X^\ast} = 1}$ is separable.
Let $\mathcal S_{X^\ast} = \set {f_n : n \in \N}$ be a countable everywhere dense subset of $S_{X^\ast}$.
For each $n \in \N$, pick $x_n \in X$ such that $\norm {x_n}_X = 1$ and:
- $\ds \size {\map {f_n} {x_n} } \ge \frac 1 2$
Let:
- $M = \span \set {x_1, x_2, \ldots}$
From Characterization of Separable Normed Vector Space, it suffices to show that:
- $M^- = X$
Suppose that $M^- \ne X$.
From Closed Linear Span is Closed Linear Subspace, we have that:
- $M^-$ is a closed linear subspace.
So, from Existence of Distance Functional, there exists $f \in X^\ast$ such that:
- $\map f x = 0$ for each $x \in M^-$ and $\norm f_{X^\ast} = 1$.
That is, $f \in S_{X^\ast}$.
Then we have:
- $\map f {x_n} = 0$ for each $n \in \N$
and so:
\(\ds \size {\map {f_n} {x_n} }\) | \(=\) | \(\ds \size {\map {f_n} {x_n} - \map f {x_n} }\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \norm {f_n - f}_{X^\ast} \norm {x_n}_X\) | Definition of Norm on Bounded Linear Functional | |||||||||||
\(\ds \) | \(=\) | \(\ds \norm {f_n - f}_{X^\ast}\) |
However since:
- $\ds \size {\map {f_n} {x_n} } \ge \frac 1 2$
for each $n \in \N$, we have:
- $\ds \norm {f_n - f}_{X^\ast} \ge \frac 1 2$
for each $n \in \N$.
So there does not exist $g \in \mathcal S_{X^\ast}$ such that:
- $\ds \norm {g - f}_{X^\ast} < \frac 1 3$
while $f \in S_{X^\ast}$.
This contradicts that $\mathcal S_{X^\ast}$ is everywhere dense in $S_{X^\ast}$.
So we must have $M^- = X$.
So, from Characterization of Separable Normed Vector Space, we have:
- $X$ is separable.