Closure of Finite Union equals Union of Closures

Theorem
Let $T$ be a topological space.

Let $n \in \N$.

Let:
 * $\forall i \in \set {1, 2, \ldots, n}: H_i \subseteq T$

Then:
 * $\ds \bigcup_{i \mathop = 1}^n \map \cl {H_i} = \map \cl {\bigcup_{i \mathop = 1}^n H_i}$

where $\map \cl {H_i}$ denotes the closure of $H_i$.

Proof
From Closure of Union contains Union of Closures we have that:


 * $\ds \bigcup_{i \mathop = 1}^n \map \cl {H_i} \subseteq \map \cl {\bigcup_{i \mathop = 1}^n H_i}$

We need now to show that:


 * $\ds \bigcup_{i \mathop = 1}^n \map \cl {H_i} \supseteq \map \cl {\bigcup_{i \mathop = 1}^n H_i}$

Let $\ds K = \bigcup_{i \mathop = 1}^n \map \cl {H_i}$ and $\ds H = \bigcup_{i \mathop = 1}^n H_i$.

From Topological Closure is Closed, all of $\map \cl {H_i}$ are closed.

So $K$ is the union of a finite number of closed sets.

So $K$ is itself closed, from Topology Defined by Closed Sets.

From Set is Subset of its Topological Closure:
 * $\forall i \in \closedint 1 n: H_i \subseteq \map \cl {H_i}$

and so $H \subseteq K$.

So from Topological Closure of Subset is Subset of Topological Closure:
 * $\map \cl H \subseteq \map \cl K$

The result follows.

Also see

 * Closure of Infinite Union may not equal Union of Closures