Alexandroff Extension of Rational Number Space is not Hausdorff
Jump to navigation
Jump to search
Theorem
Let $\struct {\Q, \tau_d}$ be the rational number space under the Euclidean topology $\tau_d$.
Let $p$ be a new element not in $\Q$.
Let $\Q^* := \Q \cup \set p$.
Let $T^* = \struct {\Q^*, \tau^*}$ be the Alexandroff extension on $\struct {\Q, \tau_d}$.
Then $T^*$ is not a $T_2$ (Hausdorff) space.
Proof
From Condition for Alexandroff Extension to be $T_2$ Space, $T^*$ is a $T_2$ space if and only if $\struct {\Q, \tau_d}$ is a locally compact Hausdorff Space.
But from Rational Number Space is not Locally Compact Hausdorff Space, $\struct {\Q, \tau_d}$ is not a locally compact Hausdorff Space.
Hence the result.
$\blacksquare$
Sources
- 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology (2nd ed.) ... (previous) ... (next): Part $\text {II}$: Counterexamples: $35$. One Point Compactification Topology: $4$