T5 Property is Hereditary
Jump to navigation
Jump to search
Theorem
Let $T = \struct {S, \tau}$ be a topological space which is a $T_5$ space.
Let $T_H = \struct {H, \tau_H}$, where $\O \subset H \subseteq S$, be a subspace of $T$.
Then $T_H$ is a $T_5$ space.
Proof
Let $T = \struct {S, \tau}$ be a $T_5$ space.
Then:
- $\forall A, B \subseteq S, \map {\cl_S} A \cap B = A \cap \map {\cl_S} B = \O: \exists U, V \in \tau: A \subseteq U, B \subseteq V, U \cap V = \O$
where $\map {\cl_S} A$ denotes the closure of $A$ in $S$.
That is:
- For any two separated sets $A, B \subseteq S$ there exist disjoint open sets $U, V \in \tau$ containing $A$ and $B$ respectively.
We have that the set $\tau_H$ is defined as:
- $\tau_H := \set {U \cap H: U \in \tau}$
Let $A, B \subseteq H$ such that $\map {\cl_H} A \cap B = A \cap \map {\cl_H} B = \O$.
That is, $A$ and $B$ are separated in $H$.
Then:
\(\ds \map {\cl_S} A \cap B\) | \(=\) | \(\ds \map {\cl_S} A \cap \paren {H \cap B}\) | as $B \subseteq H$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {\map {\cl_S} A \cap H} \cap B\) | Intersection is Associative | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {\cl_H} A \cap B\) | Closure of Subset in Subspace | |||||||||||
\(\ds \) | \(=\) | \(\ds \O\) | Assumption |
Similarly:
- $A \cap \map {\cl_S} B = \O$
So $A$ and $B$ are separated in $S$.
Because $T$ is a $T_5$ space, we have that:
- $\exists U, V \in \tau: A \subseteq U, B \subseteq V, U \cap V = \O$
It follows that:
- $\exists U \cap H, V \cap H \in \tau_H : A \subseteq U \cap H, B \subseteq V \cap H, \paren {U \cap H} \cap \paren {V \cap H} = \O$
and so the $T_5$ axiom is satisfied in $H$.
$\blacksquare$
Sources
- 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology (2nd ed.) ... (previous) ... (next): Part $\text I$: Basic Definitions: Section $2$: Separation Axioms: Functions, Products, and Subspaces