Characterization of Paracompactness in T3 Space/Lemma 8

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

Let every open cover of $T$ be even.

Let $\UU$ be an open cover of $T$.

Then:
 * there exists a $\sigma$-discrete refinement $\AA$ of $\UU$

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

By definition of even cover there exists a neighborhood $V$ of the diagonal $\Delta_X$ of $X \times X$ in the product space $\struct {X \times X, \tau_{X \times X} }$:
 * $\forall x \in X : \exists U \in \UU : \map V x = \set {y \in X : \tuple {x, y} \in V} \subseteq U$

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

In what follows subsets of $X \times X$ will be treated at times as a relation on $X \times X$.

Lemma 13
For all $n \in \N_{> 0}$, let:
 * $U_n = V_n \circ V_{n - 1}, \circ \cdots \circ V_1$

Then:
 * $\forall n \in \N_{> 0} : U_{n + 1} = V_{n + 1} \circ U_n$

Lemma 15

 * $\forall n : \set{\map {U_n} x : x \in X}$ refines $\UU$

From Zermelo's Well-Ordering Theorem, let:
 * $\preccurlyeq$ well-order $X$

For each $n \in \N_{> 0}, x \in X$, let:
 * $\map {A_n} x = \map {U_n} x \setminus \ds \bigcup_{y \preccurlyeq x} \map {U_{n + 1}} y$

For each $n \in \N_{> 0}$, let:
 * $\AA_n = \set{\map {A_n} x : x \in X}$

$\AA_n$ is a Discrete Set of Subsets

 * $\AA_n$ is a discrete set of subsets

Let:
 * $\AA = \ds \bigcup_{n \in \N, n \ne 0} \AA_n$

$\AA$ is a Cover of $X$

 * $\cdots$

$\AA$ is a Refinement of $\UU$

 * $\cdots$

It follows that $\AA$ is a $\sigma$-discrete refinement of $\UU$ by definition.