Inner Limit in Hausdorff Space by Open Neighborhoods

Notation
Notation 2: The following notation will be used for the classes of cofinite and cofinal subsets of $\N$:


 * $\mathcal{N}_\infty:= \left\{N\subset \N| \N \setminus N \text{ is finite}\right\}$
 * $\mathcal{N}_\infty^\#:= \{N\subset \N| N \text{ is infinite}\}$

Theorem
Let $\left \langle{C_n}\right \rangle_{n \in \N}$ be a sequence of sets in a Hausdorff topological space $\left({\mathcal X, \tau}\right)$.

Let $x \in \mathcal X$.

Let $\mho \left({x}\right) := \left\{ {V \in \tau:\ x \in V}\right\}$ denote the set of open neighborhoods of $x$.

Let $\mathcal N_\infty$ denote the set of cofinite subsets of $\N$:
 * $\mathcal N_\infty := \left\{ {N \subset \N: \N \setminus N \text{ is finite} }\right\}$

Then the inner limit of $\left \langle{C_n}\right \rangle_{n \in \N}$ is:


 * $\displaystyle \liminf_n \ C_n = \left\{ {x \in \mathcal X: \forall V \in \mho \left({x}\right): \exists N \in \mathcal N_\infty: \forall n \in N: C_n \cap V \ne \varnothing}\right\}$

or equivalently:


 * $\displaystyle \liminf_n \ C_n = \left\{{x \in \mathcal X: \forall V \in \mho \left({x}\right): \exists N_0 \in \N: \forall n \ge N_0: C_n \cap V \ne \varnothing}\right\}$

Proof
(1). If $x\in\liminf_n \ C_n$ then we can find a sequence $\left \langle{x_k}\right \rangle_{n \in \N}$ such that


 * $x_k\to x$ while $x_k\in C_{n_k}$

and


 * $\left \langle{n_k}\right \rangle_{k\in\N}\subseteq\N$ is a strictly increasing sequence of indices.

For any $V\in\mho\left(x\right)$ there is a $N_0\in\N$ such that for all $i\geq N_0$ it is:


 * $x_i\in V$

but also


 * $x_i\in C_{n_i}$

Thus,


 * $C_{n_i}\cap V\neq \emptyset$

Therefore $x$ is in the right-hand side set of the equation.

(2). For the reverse direction assume that $x$ belongs to the right-hand side set of the given equation. That is,


 * $\forall V\in\mho(x),\ \exists N\in \mathcal{N}_\infty, \forall n\in N: C_n\cap V\neq \emptyset$

Then, there is a strictly increasing sequence $\left \langle{n_k}\right \rangle_{k\in\N}\subseteq\N$ such that for every


 * $V\in\mho\left(x\right)$

We can find a $x_k \in C_{n_k}\cap V$.

Hence, $x_k \to x$ ( in the topology $\tau$ ).

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