Characterization of Convergence in Locally Convex Space
Theorem
Let $\Bbb F \in \set {\R, \C}$.
Let $\struct {X, \mathcal P}$ be a locally convex space over $\Bbb F$ with standard topology $\tau$.
Let $x \in X$.
Let $\sequence {x_n}_{n \mathop \in \N}$ be a sequence in $X$.
Then:
- $x_n \to x$ in $\struct {X, \tau}$
- $\map p {x_n - x} \to 0$ as $n \to \infty$ as a real sequence, for each $p \in \mathcal P$.
Proof
Necessary Condition
Suppose that:
- $x_n \to x$ in $\struct {X, \tau}$
Let $p \in \mathcal P$.
Let $\epsilon > 0$.
We aim to show that there exists $N \in \N$ such that:
- $\map p {x_n - x} < \epsilon$ for $n \ge N$
showing that:
- $\map p {x_n - x} \to 0$ as $n \to \infty$.
From the definition of a convergent sequence in a topological space, we have:
- for each $U \in \tau$ such that $x \in U$, there exists $N \in \N$ such that $x_n \in U$ for $n \ge N$.
From the definition of the standard topology $\tau$, we have:
- $U = \set {y \in X : \map p {y - x} < \epsilon}$ is open.
So there exists $N \in \N$ such that $x_n \in U$ for $n \ge N$.
That is:
- $\map p {x_n - x} < \epsilon$ for $n \ge N$.
Since $\epsilon$ was arbitrary we have:
- $\map p {x_n - x} \to 0$ as $n \to \infty$.
$\Box$
Sufficient Condition
Suppose that:
- $\map p {x_n - x} \to 0$ as $n \to \infty$ as a real sequence, for each $p \in \mathcal P$.
Let $U \in \tau$ be such that $x \in U$.
We aim to show that there exists $N \in \N$ such that $x_n \in U$ for $n \ge N$.
From Open Sets in Standard Topology of Locally Convex Space, there exists $\epsilon > 0$ and $p_1, p_2, \ldots, p_m \in \mathcal P$ such that:
- $\set {y \in X : \map {p_k} {y - x} < \epsilon \text { for each } k \in \set {1, 2, \ldots, m} } \subseteq U$
Since:
- $\map {p_k} {x_n - x} \to 0$ as $n \to \infty$
for each $k \in \set {1, 2, \ldots, m}$, there exists $N_k \in \N$ such that:
- $\map {p_k} {x_n - x} < \epsilon$ for $n \ge N_k$.
Set:
- $N = \max \set {N_1, N_2, \ldots, N_k}$
Then, for $n \ge N$, we have:
- $\map {p_k} {x_n - x} < \epsilon$ for all $k \in \set {1, 2, \ldots, m}$.
Then:
- $x_n \in U$ for $n \ge N$.
Since $U$ was an arbitrary open set containing $x$, we have:
- $x_n \to x$ in $\struct {X, \tau}$.
$\blacksquare$