Interior and Closure

Closed Set

A subset

A

of a topological space

X

is said to be closed in

X

X ∖ A

is open in

X

Note: When $X$ is known by context then we may say $A$ is closed to mean that $A$ is closed in $X$

Closed Topology

Suppose that

X

is a topological space, then the following conditions hold:

$\emptyset$ and $X$ are closed
arbitrary intersections of closed sets are closed
finite unions of closed sets are closed

The complement of $\emptyset, X$ in $X$ are $X, \emptyset$ respectively, which are both open in $X$ , making the original sets closed

Suppose that ${A_{α}}_{α \in I}$ is a collection of closed sets, then we know that $X ∖ ⋂_{α \in I} A_{α} = ⋃_{α \in I} (X ∖ A_{α})$ , and since each $A_{β}$ is closed then $X ∖ A_{β}$ is open, making $⋃_{α \in I} (X ∖ A_{α})$ an arbitrary union of open sets, and is thus open, making $X ∖ ⋂_{α \in I} A_{α}$ open

Now suppose that $I$ is finite, and we'll show that $X ∖ ⋃_{α \in I} A_{α}$ is open, we know this set is equal to $⋂_{α \in I} (X ∖ A_{α})$ , and as a finite intersection of open sets, we know it is open as well showing the original set is open

Closed in a Subspace if and only if it's an Intersection

Let

Y

be a subspace of

X

, then a set

A

is closed in

Y

if and only if it equals the intersection of a closed set of

X

with

Y

Let's start by assuming that we have a set $A = C \cap Y$ where $C$ is closed in $X$ , therefore $X ∖ C$ is open in $X$ , therefore $Y \cap (X ∖ C)$ is open in $Y$

Note that $Y \cap (X ∖ C)$ $=$ $(X \cap Y) ∖ C$ $=$ $(X \cap Y) ∖ (C \cap Y)$ $=$ $Y ∖ A$ (where the last step is justified since we know $Y \subseteq X$ ), this shows that $Y ∖ A$ is open, meaning that $A$ is closed in $Y$

Now let's assume that $A$ is closed in $Y$ , therefore $A \subseteq Y$ and $Y ∖ A$ is open in $Y$ , meaning that there is some set $U$ open in $X$ such that $Y ∖ A = Y \cap U$

Let's note that $Y ∖ (Y ∖ A)$ $=$ $(Y \cap A) \cup (Y ∖ Y) = A \cup \emptyset = A$ so that

\begin{aligned} A & = Y ∖ (Y ∖ A) \\ = Y ∖ (Y \cap U) \\ = (Y ∖ Y) \cup (Y ∖ U) \\ = Y ∖ U \\ = (X \cap Y) ∖ U \\ = Y \cap (X ∖ U) \end{aligned}

Since $X ∖ U$ is closed in $X$ this shows that $A$ equals the intersection of a closed set with $Y$ as needed.

Interior

Suppose that

A

is a subset of a topological space, then the interior of

A

is defined to as the union of all open sets contained in

A

and is denoted by

Int (A)

Closure

Suppose that

A

is a subset of a topological space, then the closure of

A

is defined to as the intersection of all closed sets containing

A

and is denoted by

\bar{A}

The Closure is Closed

Int (A)

is closed

The closure of

A

is defined as

⋂_{S \supseteq A} S

where each

S

is a closed set, to show it's closed, we must show that

X ∖ ⋂_{S \supseteq A} S

is open which we can see since it equals

⋂_{S \supseteq A} (X ∖ S)

and thus is an arbitrary union of open sets so it is open.

The Interior is Open

\bar{A}

is open

The interior is an arbitrary union of open sets, and is thus open.

Closed Supersets are Supersets of the Closure

Suppose that

\bar{A}

is the closure of

A

in a topological space, then given an closed set

A \subseteq U

we have

\bar{A} \subseteq U

TODO

Open Subsets are Subsets of the Interior

Suppose that

Int (A)

is the interior of

A

in a topological space, then given an open set

U \subseteq A

we have

U \subseteq Int (A)

TODO

Interior is Smaller, Closure is Bigger

Suppose that

A

is a subset of a topological space, then we have

Int (A) \subseteq A \subseteq \bar{A}

TODO

Open Sets Equal their Interior

Suppose that

A

is a subset of a topological space

X

, then

A

is open if and only if

A = Int (A)

Suppose that $A$ is the collection of open sets which are also subsets of $A$ , since $A$ is open then $A \in A$ , then $\bar{A} = ⋃ A = ⋃ (A ∖ {A}) \cup A$ , since each element in $A ∖ {A}$ is a subset of $A$ , then so is the union, thus $⋃ (A ∖ {A}) \cup A = A$ as needed.

The other direction is easier, suppose that $A = Int (A)$ , then $Int (A)$ is open, then so is $A$ as needed.

Closed Sets Equal their Closure

A

is a subset of a topological space

X

, then

A

is closed if and only if

A = \bar{A}

Suppose that $A$ is the collection of closed sets which are also supersets of $A$ , then $\bar{A} = ⋂ A = ⋂ (A ∖ {A}) \cap A$ , and since each element in $A ∖ {A}$ is a superset of $A$ , then so is it's intersection, therefore $⋂ (A ∖ {A}) \cap A = A$

Now suppose that $A = \bar{A}$ , since $\bar{A}$ is closed then we know $A$ is closed

Closure in a Subspace is an Intersection

Let

Y

be a subspace of

X

and

A \subseteq Y

, then suppose that

\bar{A}

is the closure of

A

X

, then the closure of

A

Y

equals

\bar{A} \cap Y

Set $B$ equal to the closure of $A$ in $Y$ , since $\bar{A}$ is closed in $X$ , then we know that $\bar{A} \cap Y$ is closed in $Y$ , also note that $A \subseteq \bar{A} \cap Y$ because we know $A \subseteq \bar{A}$ and $A \subseteq Y$ so we can conclude $B \subseteq (\bar{A} \cap Y)$

On the other hand, we know that $B$ is closed in $Y$ , thus $B = C \cap Y$ for some set $C$ closed in $X$ , we recall that $A \subseteq B$ so that $A \subseteq (C \cap Y)$ , therefore $A \subseteq C$ showing that $C$ is a closed set containing $A$ , thus $\bar{A} \subseteq C$ so that $(\bar{A} \cap Y) \subseteq (C \cap Y) = B$ , as needed.

Neighborhood

In a topological space

X

and a point

x \in X

then if

U

is an open set containing

x

then we say that

U

is a neighborhood of

x

Closure Intersection Equivalence

x \in \bar{A}

if and only if neighborhood of

x

intersects

A

We prove the contrapositive, that is $x \notin \bar{A}$ iff there is some open set $U$ containing $x$ that doesn't intersect $A$

Suppose that $x \notin \bar{A}$ , since $\bar{A}$ is closed, then $U = X ∖ \bar{A}$ must be open and containing $x$ and since $A \subseteq \bar{A}$ , we know $U$ doesn't intersect $A$ , as needed.

Now the reverse direction: assuming that we have some open set $U$ containing $x$ that doesn't intersect $A$ , we have $A \subseteq X ∖ U$ and $X ∖ U$ is closed therefore $\bar{A} \subseteq X ∖ U$ , so that $\bar{A} \cap U = \emptyset$ , since we know $x \in U$ we know that $x \notin \bar{A}$ as needed.

Closure Basis Intersection Equivalence

Suppose that a basis

B

generates a topology

T

, then

x \in \bar{A}

if and only if every basis element

B

containing

x

intersects

A

Suppose that $x \in \bar{A}$ , therefore every open set containing $x$ intersects $A$ , now suppose that $B \in B$ , since it is open, then we know that $B$ intersects $A$

Now suppose that every basis element containing $x$ intersects $A$ , and we'd like to show that $x \in \bar{A}$ which is equivalent to every open set containing $x$ intersecting $A$ , so suppose that $U \in T$ since $B$ generates $T$ , then there is some $B \in B$ such that $x \in B \subseteq U$ since $A$ intersects $B$ then it also intersects $U$ as needed.