Characterization of Paracompactness in T3 Space/Lemma 20

Theorem
Let $T = \struct{X, \tau}$ be a $T_3$ Space.

Let every open cover of $T$ be even.

Let $X \times X$ denote the cartesian product of $X$ with itself.

Let $\tau_{X \times X}$ denote the product topology on $X \times X$.

Let $T \times T$ denote the product space $\struct {X \times X, \tau_{X \times X} }$.

Let $U$ be a neighborhood of the diagonal $\Delta_X$ of $X \times X$ in $T \times T$.

Then there exists a neighborhood $W$ of the diagonal $\Delta_X$ in $T \times T$:
 * $W$ is symmetric as a relation on $X \times X$
 * the composite relation $W \circ W$ is a subset of $U$, that is, $W \circ W \subseteq U$

Proof
Let:
 * $\VV = \set{V \in \tau : V \times V \subseteq U}$

From User:Leigh.Samphier/Topology/Neighborhood of Diagonal induces Open Cover:
 * $\VV$ is an open cover of $T$

We have, $\VV$ is even.

By definition of even cover, there exists a neighborhood $R$ of the diagonal $\Delta_S$ in $T \times T$:
 * $\forall x \in X : \exists V \in \VV : \map R x = \set {y \in X : \tuple {x, y} \in V} \subseteq V$

where:
 * $R$ is seen as a relation on $X \times X$
 * $\map R x$ denotes the image of $x$ under $R$.

From Subset Relation is Transitive:
 * $\forall x \in X : \map R x \times \map R x \subseteq U$

Let $W = R \cap R^{-1}$, where $R^{-1}$ is the inverse relation of $R$ on $X \times X$.

Lemma 15

 * $W$ is a neighborhood $W$ of the diagonal $\Delta_X$ in $T \times T$:
 * $W$ is a symmetric relation on $X \times X$
 * $\forall x \in X : \map W x \times \map W x \subseteq U$

Lemma 16

 * $W \circ W = \ds \bigcup_{x \in X} \map W x \times \map W x$

From Union of Subsets is Subset:
 * $W \circ W \subseteq U$