Equivalence of Definitions of Baire Space

Definition

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

The following definitions of the concept of Baire Space in the context of Topology are equivalent:

$T$ is a Baire space if and only if:

$(1): \quad$ The intersection of any countable set of open sets of $T$ which are everywhere dense is everywhere dense.
$(2): \quad$ The interior of the union of any countable set of closed sets of $T$ which are nowhere dense is empty.
$(3): \quad$ Whenever the union of any countable set of closed sets of $T$ has an interior point, then one of those closed sets must have an interior point.
$(4): \quad$ The union of any countable set of closed sets of $T$ whose interiors are empty also has an interior which is empty.

Proof

First, let:

$H^\circ$ denote the interior of any $H \subseteq S$
$H^-$ denote the closure of any $H \subseteq S$.

$(2) \iff (4)$

We have that a Closed Set Equals its Closure.

By definition, a subset $H$ is nowhere dense if and only if the interior of its closure is empty.

Hence we see that $(2)$ and $(4)$ are saying the same thing using different words.

$\Box$

$(4) \iff (3)$

$(4) \implies (3)$

Let $T$ be a topological space such that:

The union of any countable set of closed sets of $T$ whose interiors are empty also has an empty interior.

That is, let $(4)$ hold.

Let $\UU$ be a countable set of closed sets of $T$.

Let $\ds \bigcup \UU$ be their union.

Suppose $\exists U \in \UU$ such that $\ds \exists x \in \paren {\bigcup \UU}^\circ$.

That is, let $x$ be an interior point of $\ds \bigcup \UU$.

Then by hypothesis and the Rule of Transposition $\exists U \in \UU: x \in U^\circ$.

That is, $x$ is an interior point of $U$.

That is, $(3)$ holds.

$\Box$

$(3) \implies (4)$

Let $T$ be a topological space such that:

Whenever the union of any countable set of closed sets of $T$ has an interior point, then one of those closed sets must have an interior point.

That is, let $(3)$ hold.

Let $\UU$ a countable set of closed sets of $T$.

Suppose that $\forall U \in \UU: U^\circ = \O$.

Then by hypothesis and the Rule of Transposition $\not \exists x \in \ds \bigcup \UU$.

That is, $\ds \bigcup \UU = \O$.

That is, $(4)$ holds.

$\Box$

$(4) \iff (1)$

$(4) \implies (1)$

Let $T$ be a topological space such that:

The union of any countable set of closed sets of $T$ whose interiors are empty also has an empty interior.

That is, let $(4)$ hold.

Let $\UU$ be a countable set of open sets of $T$ such that:

$\forall U \in \UU: U^- = S$

That is, all of $U$ are everywhere dense.

We have that:

 $\ds U^-$ $=$ $\ds S$ $\ds \leadstoandfrom \ \$ $\ds S \setminus U^-$ $=$ $\ds \O$ Relative Complement with Self is Empty Set $\ds \leadstoandfrom \ \$ $\ds \paren {S \setminus U}^\circ$ $=$ $\ds \O$ Complement of Interior equals Closure of Complement

That is, by definition, $S \setminus U$ is nowhere dense.

By definition of closed set we have that $S \setminus U$ is closed.

Now consider $\ds \bigcup_{U \mathop \in \UU} \paren {S \setminus U}$.

We have that:

 $\ds \paren {\bigcup_{U \mathop \in \UU} \paren {S \setminus U} }^\circ$ $=$ $\ds \O$ as $T$ satisfies condition $(4)$ $\ds \leadstoandfrom \ \$ $\ds \paren {S \setminus \bigcap \UU}^\circ$ $=$ $\ds \O$ De Morgan's Laws: Difference with Intersection $\ds \leadstoandfrom \ \$ $\ds S \setminus \paren {\paren {\bigcap \UU}^-}$ $=$ $\ds \O$ Complement of Interior equals Closure of Complement $\ds \leadstoandfrom \ \$ $\ds \paren {\bigcap \UU}^-$ $=$ $\ds S$ Relative Complement of Empty Set

That is, by definition, $\bigcap \UU$ is everywhere dense.

That is, $(1)$ holds.

$\Box$

$(1) \implies (4)$

Let $T$ be a topological space such that:

The intersection of any countable set of open sets of $T$ which are everywhere dense is everywhere dense.

That is, let $(1)$ hold.

Let $\VV$ be a countable set of closed sets of $T$ such that:

$\forall V \in \VV: V^\circ = \O$

Then:

 $\ds V^\circ$ $=$ $\ds \O$ $\ds \leadstoandfrom \ \$ $\ds S \setminus V^\circ$ $=$ $\ds S$ Relative Complement of Empty Set $\ds \leadstoandfrom \ \$ $\ds \paren {S \setminus V}^-$ $=$ $\ds S$ Complement of Interior equals Closure of Complement

That is, by definition, $S \setminus V$ is an open set of $T$ which is everywhere dense.

Now consider $\ds \bigcap_{V \mathop \in \VV} \paren {S \setminus V}$.

We have that:

 $\ds \paren {\bigcap_{V \mathop \in \VV} \paren {S \setminus V} }^-$ $=$ $\ds S$ as $T$ satisfies condition $(1)$ $\ds \leadstoandfrom \ \$ $\ds \paren {S \setminus \bigcup \VV}^-$ $=$ $\ds S$ De Morgan's Laws: Difference with Union $\ds \leadstoandfrom \ \$ $\ds S \setminus \paren {\paren {\bigcup \VV}^\circ}$ $=$ $\ds S$ Complement of Interior equals Closure of Complement $\ds \leadstoandfrom \ \$ $\ds \paren {\bigcup \VV}^\circ$ $=$ $\ds \O$ Relative Complement with Self is Empty Set

That is, $(4)$ holds.

$\Box$

All conditions have been shown to be equivalent.

$\blacksquare$