diff --git a/src/fa/duality/definitions.tex b/src/fa/duality/definitions.tex index f67f9b0..722fb5c 100644 --- a/src/fa/duality/definitions.tex +++ b/src/fa/duality/definitions.tex @@ -14,6 +14,15 @@ In the context of a dual system, $E$ and $F$ are identified as subspaces of each others' algebraic duals. \end{definition} +\begin{definition}[Norming Duality] +\label{definition:norming-duality} + Let $K \in \RC$ and $\dpn{E, F}{\lambda}$ be a duality of normed vector spaces over $K$, then $\dpn{E, F}{\lambda}$ is \textbf{norming} if: + \begin{enumerate} + \item For each $x \in E$, $\norm{x}_E = \sup_{y \in F, \norm{y}_F \le 1}\dpn{x, y}{\lambda}$. + \item For each $y \in F$, $\norm{y}_F = \sup_{x \in E, \norm{x}_E \le 1}\dpn{x, y}{\lambda}$. + \end{enumerate} +\end{definition} + \begin{definition}[Weak Topology] \label{definition:duality-weak-topology} Let $K \in \RC$ and $\dpn{E, F}{\lambda}$ be a duality over $K$, then the weak topology generated by $F$, denoted $\sigma(E, F)$, is the \textbf{weak topology} of the duality on $E$. diff --git a/src/fa/norm/separable.tex b/src/fa/norm/separable.tex index 9a51796..93a11f1 100644 --- a/src/fa/norm/separable.tex +++ b/src/fa/norm/separable.tex @@ -3,22 +3,26 @@ \begin{proposition} \label{proposition:separable-dual} - Let $E$ be a separable normed vector space, then $E^*$ is separable with respect to the weak*-topology. + Let $K \in \RC$ and $\dpn{E, F}{\lambda}$ be a duality of normed vector spaces over $K$ with $E$ being separable, then + \begin{enumerate} + \item The closed unit ball $S = \bracsn{y \in F|\ \norm{y}_{F} \le 1}$ is separable with respect to the $\sigma(F, E)$-topology. + \item If the duality is norming, then there exists $\seq{y_n} \subset F$ such that for each $x \in E$, $\norm{x}_E = \sup_{n \in \natp}\dpn{x, y_n}{\lambda}$. + \end{enumerate} \end{proposition} \begin{proof} - Let $\seq{x_n} \subset E$ be a dense subset and $S = \bracsn{\phi \in E^*| \norm{\phi}_{E^*} \le 1}$. For each $N \in \natp$, let + Let $\seq{x_n} \subset E$ be a dense subset. For each $N \in \natp$, let \[ - T_N: S \to \real^N \quad \phi \mapsto (\dpn{x_1, \phi}{E}, \cdots, \dpn{x_N, \phi}{E}) + T_N: S \to \real^N \quad y \mapsto (\dpn{x_1, y}{E}, \cdots, \dpn{x_N, y}{E}) \] - Since $\real^N$ is separable, $T_N(S)$ is separable by \autoref{proposition:separable-metric-space}. Thus there exists $\bracs{\phi_{N, k}}_{k = 1}^\infty \subset S$ such that $\bracs{T_N\phi_{N, k}}_{k = 1}^\infty$ is dense in $T_N(S)$. + Since $\real^N$ is separable, $T_N(S)$ is separable by \autoref{proposition:separable-metric-space}. Thus there exists $\bracs{y_{N, k}}_{k = 1}^\infty \subset S$ such that $\bracs{T_Ny_{N, k}}_{k = 1}^\infty$ is dense in $T_N(S)$. - Let $\phi \in S$, then for each $N \in \natp$, there exists $k_N \in \natp$ such that for each $1 \le n \le N$, + Let $y \in S$, then for each $N \in \natp$, there exists $k_N \in \natp$ such that for each $1 \le n \le N$, \[ |\dpn{x_n, \phi_{N, k_N}}{E} - \dpn{x_n, \phi}{E}| \le \frac{1}{N} \] - Thus for each $N \in \natp$, $\dpn{x_n, \phi_{N, k_N}}{E} \to \dpn{x_n, \phi}{E}$ as $N \to \infty$. Since $\phi_{N, k_N} \to \phi$ pointwise on a dense subset of $E$, and $\bracsn{\phi_{N, k_N}|N \in \natp} \subset S$ is uniformly equicontinuous, $\phi_{N, k_N} \to \phi$ in the weak*-topology by \autoref{proposition:strong-operator-dense}. + Thus for each $N \in \natp$, $\dpn{x_n, y_{N, k_N}}{E} \to \dpn{x_n, y}{E}$ as $N \to \infty$. Since $\phi_{N, k_N} \to \phi$ pointwise on a dense subset of $E$, and $\bracsn{y_{N, k_N}|N \in \natp} \subset S$ is uniformly equicontinuous, $\phi_{N, k_N} \to \phi$ in the $\sigma(F, E)$-topology by \autoref{proposition:strong-operator-dense}. \end{proof} \begin{proposition}