Characterization of Boundary by Open Sets
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Theorem
Let $T = \struct {S, \tau}$ be a topological space.
Let $A$ be a subset of $T$.
Let $x$ be a point of $T$.
Then $x \in \partial A$ if and only if:
- for every open set $U$ of $T$:
- if $x \in U$
- then $A \cap U \ne \O$ and $\relcomp S A \cap U \ne \O$
where:
- $\relcomp S A = S \setminus A$ denotes the complement of $A$ in $S$
- $\partial A$ denotes the boundary of $A$.
Proof
Sufficient Condition
Let $x \in \partial A$.
Then by Boundary is Intersection of Closure with Closure of Complement:
- $x \in \paren {\relcomp S A}^-$ and $x \in A^-$
where $A^-$ denotes the closure of $A$.
Hence by Condition for Point being in Closure, for every open set $U$ of $T$:
- $x \in U \implies A \cap U \ne \O$
and:
- $x \in U \implies \relcomp S A \cap U \ne \O$
Necessary Condition
Let $x$ be such that for every open set $U$ of $T$:
- if $x \in U$
- then $A \cap U \ne \O$ and $\relcomp S A \cap U \ne \O$.
Then by Condition for Point being in Closure:
- $x \in \paren {\relcomp S A}^-$ and $x \in A^-$.
Hence by Boundary is Intersection of Closure with Closure of Complement:
- $x \in \partial A$
$\blacksquare$
Sources
- Mizar article TOPS_1:28