Union of Topologies on Singleton or Doubleton is Topology
Theorem
Let $S$ be a set which is either a singleton or a doubleton.
Let $\family {\tau_i}_{i \mathop \in I}$ be an arbitrary non-empty indexed set of topologies on $S$.
Then $\tau := \ds \bigcup_{i \mathop \in I} {\tau_i}$ is also a topology on $S$.
Proof
Let $S$ be a singleton.
Let $S = \set a$.
From Topology on Singleton is Indiscrete Topology, the only possible topology on $S$ is the indiscrete topology.
From Set Union is Idempotent, the union of any number of indiscrete topologies on $S$ is the indiscrete topology.
Thus the union of any number of topologies on a singleton is a topology on that singleton.
$\Box$
Let $S$ be a doubleton.
Let $S = \set {a, b}$.
By Topologies on Doubleton, there are $4$ possible different topologies on $S$:
\(\ds \tau_1\) | \(=\) | \(\ds \set {\O, \set {a, b} }\) | Indiscrete Topology | |||||||||||
\(\ds \tau_2\) | \(=\) | \(\ds \set {\O, \set a, \set {a, b} }\) | Sierpiński Topology | |||||||||||
\(\ds \tau_3\) | \(=\) | \(\ds \set {\O, \set b, \set {a, b} }\) | Sierpiński Topology | |||||||||||
\(\ds \tau_4\) | \(=\) | \(\ds \set {\O, \set a, \set b, \set {a, b} }\) | Discrete Topology |
It remains to show that every union of elements in $\set {\tau_1, \tau_2, \tau_3, \tau_4}$ is a topology on $S$.
By inspection, the following statements are seen to hold:
- $\tau_1 \cup \tau_2 = \tau_2$
- $\tau_1 \cup \tau_3 = \tau_3$
- $\tau_1 \cup \tau_4 = \tau_4$
- $\tau_2 \cup \tau_3 = \tau_4$
- $\tau_2 \cup \tau_4 = \tau_4$
- $\tau_3 \cup \tau_4 = \tau_4$
Now let $\family {\tau_i}_{i \mathop \in I}$ be an arbitrary non-empty indexed set of topologies for a set $S$.
By the above, if $\tau_4$ is one of the $\tau_i$:
- $\ds \tau = \bigcup_{i \mathop \in I} \tau_i = \tau_4$
So assume that $\tau_4$ is not one of the $\tau_i$.
If both $\tau_2$ and $\tau_3$ are in $\family {\tau_i}_{i \mathop \in I}$, then by the above:
- $\ds \tau = \bigcup_{i \mathop \in I} \tau_i = \tau_4$
Now suppose that $\tau_3$ is in $\family {\tau_i}_{i \mathop \in I}$ and $\tau_2$ is not.
Then by the above it follows that:
- $\tau = \tau_3$
Now suppose that $\tau_2$ is in $\family {\tau_i}_{i \mathop \in I}$ and $\tau_3$ is not.
Then by the above it follows that:
- $\tau = \tau_2$
Finally, assume that $\tau_2$ and $\tau_3$ are not in $\family {\tau_i}_{i \mathop \in I}$.
Then the only topology in $\family {\tau_i}_{i \mathop \in I}$ is $\tau_1$.
In this, we find:
- $\tau = \tau_1$
Hence the result.
$\blacksquare$
Also see
- Union of Topologies is not necessarily Topology, which shows that when $\size S \ge 3$ this result is false.