Characterization of Paracompactness in T3 Space/Lemma 20

Theorem
Let $T = \struct{X, \tau}$ be a topological space.

Let every open cover of $T$ be even.

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

Let $U$ be a neighborhood of the diagonal $\Delta_X$ of $X \times X$ in the product space $\struct {X \times X, \tau_{X \times X} }$.

Then there exists an open neighborhood $W$ of the diagonal $\Delta_X$ in $\struct {X \times X, \tau_{X \times X} }$:
 * $W$ is symmetric as a relation on $X \times X$, that is, $W = W^{-1}$
 * 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.

From User:Leigh.Samphier/Topology/Charaterization of Even Cover, there exists an open neighborhood $R$ of the diagonal $\Delta_X$ in $\struct {X \times X, \tau_{X \times X} }$:
 * $\set{\map R x : x \in S}$ is a refinement of $\VV$

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

By definition of refinement of cover:
 * $\forall x \in X : \exists V \in \VV : \map R x \subseteq V$

From Cartesian Product of Subsets:
 * $\forall x \in X : \exists V \in \VV : \map R x \times \map R x \subseteq V \times V$

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 $X \times X$.

From User:Leigh.Samphier/Topology/Inverse of Open Set in Product Space is Open in Inverse Product Space:
 * $R^{-1}$ is open in $\struct {X \times X, \tau_{X \times X}}$

By :
 * $W$ is open in $\struct {X \times X, \tau_{X \times X}}$

From Inverse of Reflexive Relation is Reflexive:
 * $R^{-1}$ is reflexive

From Intersection of Reflexive Relations is Reflexive:
 * $W$ is reflexive

By definition of reflexive:
 * $W$ is an open neighborhood of the diagonal $\Delta_X$.

From Intersection of Relation with Inverse is Symmetric Relation:
 * $W$ is a symmetric relation on $X \times X$

We have:

By definition of subset:
 * $\forall x \in X : \map W x \times \map W x \subseteq U$

From Composition of Symmetric Relation with Itself is Union of Products of Images:
 * $W \circ W = \ds \bigcup_{x \in X} \map W x \times \map W x$

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