Added more bits on bornologic spaces.
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Bokuan Li
2026-05-01 19:27:38 -04:00
parent 0ef220bba5
commit dcf11fb978
2 changed files with 31 additions and 11 deletions

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@@ -22,6 +22,21 @@
(2) $\Rightarrow$ (1): Let $\rho$ be the \hyperref[gauge]{definition:gauge} of $U$, then for any $B \subset E$ bounded, there exists $R > 0$ such that $B \subset RU$. In which case, $\rho(B) \subset [0, R]$.
\end{proof}
\begin{proposition}
\label{proposition:bornologic-bounded}
Let $E$ be a bornologic space, $F$ be a locally convex space, and $T \in \hom(E; F)$, then the following are equivalent:
\begin{enumerate}
\item $T$ is continuous.
\item $T$ is sequentially continuous.
\item $T$ is bounded.
\end{enumerate}
\end{proposition}
\begin{proof}
(2) $\Rightarrow$ (3): Let $B \subset E$ be a bounded set, $\seq{x_n} \subset E$, and $\seq{\lambda_n} \subset K$ such that $\lambda_n \to 0$ as $n \to \infty$, then $\lambda_n x_n \to 0$ as $n \to \infty$. By sequential continuity of $T$, $T(\lambda_n x_n) \to 0$ as $n \to \infty$ as well. Thus $T(B)$ is also bounded.
(3) $\Rightarrow$ (1): Let $\rho: F \to [0, \infty)$ be a continuous seminorm, then $\rho \circ T$ is a seminorm on $E$ that is bounded on bounded sets. Since $E$ is bornologic, $\rho \circ T$ is continuous. Therefore $T$ is continuous by \autoref{proposition:tvs-convex-morphism}.
\end{proof}
\begin{proposition}
\label{proposition:metrisable-bornologic}
Let $E$ be a metrisable locally convex space, then $E$ is bornologic.
@@ -31,20 +46,14 @@
\end{proof}
\begin{proposition}
\label{proposition:bornologic-bounded}
Let $E$ be a bornologic space, $F$ be a locally convex space, and $T \in \hom(E; F)$, then the following are equivalent:
\begin{enumerate}
\item $T$ is continuous.
\item $T$ is bounded.
\end{enumerate}
\label{proposition:bornologic-limit}
Let $\seqi{E}$ be bornologic spaces over $K \in \RC$, $E$ be a vector space over $K$, and $\seqi{T}$ such that $T_i \in \hom(E_i; E)$ for all $i \in I$, then $E$ equipped with the inductive topology is bornolgic.
\end{proposition}
\begin{proof}
(1) $\Rightarrow$ (2): By \autoref{proposition:continuous-bounded}.
(2) $\Rightarrow$ (1): Let $\rho: F \to [0, \infty)$ be a continuous seminorm, then $\rho \circ T$ is a seminorm on $E$ that is bounded on bounded sets. Since $E$ is bornologic, $\rho \circ T$ is continuous. Therefore $T$ is continuous by \autoref{proposition:tvs-convex-morphism}.
Let $\rho: E \to [0, \infty)$ be a seminorm on $E$ that is bounded on all bounded sets. For each $i \in I$ and $B \subset E_i$ bounded, $T_i(B)$ is bounded by \autoref{proposition:bornologic-bounded}, and $\rho \circ T_i(B)$ is bounded by assumption. Thus for every $i \in I$, $\rho \circ T_i$ is continuous, so $\rho$ is continuous by (4) of \autoref{definition:lc-inductive}.
\end{proof}
\begin{proposition}
\label{proposition:bornologic-continuous-complete}
Let $E$ be a bornologic space and $F$ be a complete separated locally convex space, then $L_b(E; F)$ is complete. In particular, $E^*$ equipped with the topology of bounded convergence is complete.

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\begin{definition}[Bounded]
\label{definition:bounded}
Let $E$ be a TVS over $K \in \RC$ and $B \subset E$, then $B$ is \textbf{bounded} if for every $U \in \cn(0)$, there exists $\lambda \in K$ such that $\lambda U \supset B$. The collection $B(E)$ is the set of all bounded sets of $E$.
Let $E$ be a TVS over $K \in \RC$ and $B \subset E$, then the following are equivalent:
\begin{enumerate}
\item For every $U \in \cn(0)$, there exists $\lambda \in K$ such that $\lambda U \supset B$.
\item For every $\seq{x_n} \subset B$ and $\seq{\lambda_n} \subset K$ such that $\lambda_n \to 0$, $\lambda_n x_n \to 0$ as $n \to \infty$.
\end{enumerate}
If the above holds, then $B$ is \textbf{bounded}. The collection $B(E)$ is the set of all bounded sets of $E$.
\end{definition}
\begin{proof}
(1) $\Rightarrow$ (2): Let $U \in \cn_E(0)$ be circled, then there exists $k \in \natp$ such that $kU \supset B$. Since $\lambda_n \to 0$ as $n \to \infty$, there exists $N \in \natp$ such that $|\lambda_n| \le 1/k$ for all $n \ge N$. In which case, $\lambda_n x_n \in \lambda_n B \subset U$ for all $n \ge N$.
$\neg (1) \Rightarrow \neg (2)$: Let $U \in \cn_E(0)$ be circled such that $B \not\subset nU$ for all $n \in \natp$. For each $n \in \natp$, let $x_n \in B \setminus nU$, then $x_n/n \not\in U$ for all $n \in \natp$.
\end{proof}
\begin{proposition}[{{\cite[I.5.1]{SchaeferWolff}}}]
\label{proposition:bounded-operations}