Inner Limit in Hausdorff Space by Open Neighborhoods
Theorem
Let $\sequence {C_n}_{n \mathop \in \N}$ be a sequence of sets in a Hausdorff topological space $\struct {\XX, \tau}$.
Let $x \in \XX$.
Let $\map \mho x := \set {V \in \tau:\ x \in V}$ denote the set of open neighborhoods of $x$.
Let $\NN_\infty$ denote the set of cofinite subsets of $\N$:
- $\NN_\infty := \set {N \subset \N: \N \setminus N \text{ is finite} }$
Then the inner limit of $\sequence {C_n}_{n \mathop \in \N}$ is:
- $\ds \liminf_n C_n = \set {x \in \XX: \forall V \in \map \mho x: \exists N \in \NN_\infty: \forall n \in N: C_n \cap V \ne \O}$
or equivalently:
- $\ds \liminf_n C_n = \set {x \in \XX: \forall V \in \map \mho x: \exists N_0 \in \N: \forall n \ge N_0: C_n \cap V \ne \O}$
Proof
If $x \in \liminf_n C_n$ then there exist a sequence $\sequence {x_k}_{n \mathop \in \N}$ such that $x_k \to x$ while:
- $x_k \in C_{n_k}$
and
- $\sequence {n_k}_{k \mathop \in \N} \subseteq \N$ is a strictly increasing sequence of indices.
For any $V \in \map \mho x$ there exists $N_0 \in\N$ such that for all $i \ge N_0$:
- $x_i \in V$
and:
- $x_i \in C_{n_i}$
Thus:
- $C_{n_i} \cap V \ne \O$
Therefore $x \in \set {x \in \XX: \forall V \in \map \mho x: \exists N_0 \in \N: \forall n \ge N_0: C_n \cap V \ne \O}$.
$\Box$
Let $x \in \set {x \in \XX: \forall V \in \map \mho x: \exists N \in \NN_\infty: \forall n \in N: C_n \cap V \ne \O}$.
Thus:
- $\forall V \in \map \mho x: \exists N \in \NN_\infty: \forall n \in N: C_n \cap V \ne \O$
Then there exists a strictly increasing sequence:
- $\sequence {n_k}_{k \mathop \in \N} \subseteq \N$
such that for every $V \in \map \mho x$:
- $\exists x_k \in C_{n_k} \cap V$.
Hence $x_k \to x$ in the topology $\tau$.
$\blacksquare$
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Also see
- Inner Limit of Sequence of Sets in Normed Space described via the point-to-set distance function induced by the norm of the space
- Inner Limit in Normed Spaces by Open Balls
- Inner Limit in Hausdorff Space by Set Closures
- Inner Limit is Closed Set