11.10 Locally Convex Spaces of Linear Maps

Proposition 11.10.1.label Let $T$ be a set, $\sigma \subset 2^{T}$ be an ideal, $E$ be a locally convex space over $K$, and $\cf \subset E^{T}$ be a subspace such that

  1. (B)

    For each $f \in \cf$ and $S \in \sigma$, $f(S) \subset E$ is bounded.

For each $S \in \sigma$ and continuous seminorm $\rho: E \to [0, \infty)$, let

\[\rho_{S}: E^{T} \to [0, \infty] \quad f \mapsto \sup_{x \in S}\rho(f(x))\]

then the $\sigma$-uniform topology on $\cf$ is induced by seminorms of the form $\rho_{S}$, where $\rho$ is a continuous seminorm on $E$, and $S \in \sigma$. In which case, the $\sigma$-uniform topology on $\cf$ is locally convex.

Proof. By Proposition 10.12.1 and Proposition 7.2.3.$\square$

Definition 11.10.2 (Saturated Ideal).label Let $E$ be a locally convex space over $K \in \RC$ and $\sigma \subset 2^{E}$ be an ideal, then $\sigma$ is saturated if:

  1. (1)

    For each $\lambda \in K$ and $S \in \sigma$, $\lambda S \in \sigma$.

  2. (2)

    For each $S \in \sigma$, $\ol{\aconv}(S) \in \sigma$.

For any ideal $\sigma \subset 2^{E}$, the smallest saturated ideal $\ol \sigma$ containing it is the saturated hull of $\sigma$.

Lemma 11.10.3.label Let $E$, $F$ be locally convex spaces over $K \in \RC$, $\sigma \subset 2^{E}$ be an ideal, and $\ol \sigma$ be its saturated hull, then the $\sigma$-uniformity and $\ol \sigma$-uniformity on $L(E; F)$ coincide.

Proof. Let $\tau \subset \ol \sigma$ be the collection of sets such that for each $S \in \tau$ and $U \in \cn_{F}(0)$,

\[N(S, U) = \bracs{(S, T) \in L(E; F)| (S - T)(S) \subset U}\]

is an entourage in the $\sigma$-uniformity.

For each $S \in \tau$, $U \in \cn_{F}(0)$, and $\lambda \in K$ with $\lambda \ne 0$,

\begin{align*}N(\lambda S, U)&= \bracs{(S, T) \in L(E; F)| (S - T)(\lambda S) \subset U}\\&= \bracs{(S, T) \in L(E; F)| (S - T)(S) \subset \lambda^{-1}U}\end{align*}

is another entourage in the $\sigma$-uniformity. If $\lambda = 0$, then $N(\lambda S, U) = L(E; F)$, which is also an entourage.

Now, let $S \in \tau$ and $U \in \cn_{F}(0)$ be convex and circled, then by Proposition 5.5.3,

\begin{align*}N(\ol{\aconv}(S), U)&= \bracs{(S, T) \in L(E; F)| (S - T)(\ol{\aconv}(S)) \subset U}\\&\supset \bracs{(S, T) \in L(E; F)| \overline{(S - T)(\aconv(S))} \subset U}\\&= \bracs{(S, T) \in L(E; F)| \ol{\aconv}{(S - T)(S)} \subset U}\end{align*}

so $N(\ol{\aconv}(S), U)$ contains an entourage in the $\sigma$-uniformity.

Since $\tau$ is a saturated ideal that contains $\sigma$, $\tau = \ol \sigma$. Therefore the $\sigma$-uniformity and $\ol \sigma$-uniformity on $L(E; F)$ coincide.$\square$

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