Fortissimo Space is Excluded Point Space with Countable Complement Space

Theorem
Let $T = \struct {S, \tau_p}$ be a Fortissimo space.

Then $\tau_p$ is the minimal topology that is generated by the excluded point topology and the countable complement topology.

Proof
Let $T_1 = \struct {S, \tau_1}$ be the excluded point space on $S$ from $p$.

Let $T_2 = \struct {S, \tau_2}$ be the countable complement space on $S$.

By definition:
 * $\tau_1 = \set {H \subseteq S: p \in \relcomp S H} \cup \set S$


 * $\tau_2 = \leftset {H \subseteq S: \relcomp S H}$ is countable $\rightset{} \cup \set \O$

By definition of Fortissimo space, we have:


 * $U \in \tau_1 \implies U \in \tau_p$


 * $U \in \tau_2 \implies U \in \tau_p$

So $\tau_1 \cup \tau_2 \subseteq \tau_p$.

Similarly:
 * $U \in \tau_p \implies U \in \tau_1 \lor U \in \tau_2$

and so $\tau_p \subseteq \tau_1 \cup \tau_2$.

So $\tau_p = \tau_1 \cup \tau_2$ and the result follows from Union is Smallest Superset.