Tychonoff's Theorem/General Case/Proof 2/Lemma 3
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Lemma
Let $X$ be a set.
We say that a family $\CC$ of subsets of $X$ has the "finite intersection property" if:
- $\ds \bigcap_{F \in \SS} F \ne \O$
for all finite subsets $\SS \subseteq \CC$.
Let:
- $\ds S = \set {\CC \subseteq \AA : \AA \text { has the finite intersection property} }$
Let $\AA$ be a $\subseteq$-maximal element of $S$ given by Lemma 1.
Let $A \subseteq X$ be such that:
- $A \cap B \ne \O$
for all $B \in \AA$.
Then $A \in \AA$.
Proof
Let $\set {C_1, \ldots, C_n}$ be a finite subset of $\AA$.
By Lemma 2, we have:
- $\ds \bigcap_{i \mathop = 1}^n C_i \ne \O$
Hence, by hypothesis, we have:
- $\ds A \cap \bigcap_{i \mathop = 1}^n C_i \ne \O$
So $\AA \cup \set A \in S$ has the finite intersection property.
Since $\AA$ is $\subseteq$-maximal in $S$, we have:
- $\AA \cup \set A = \AA$
so that $A \in \AA$.
$\blacksquare$