Basis for Element of Real Number Line
Jump to navigation
Jump to search
Theorem
Let $M = \struct {\R, d}$ denote the real number line with the usual (Euclidean) metric.
Let $a \in \R$ be a point in $M$.
Then the set of all open intervals containing $a$ is a basis for the neighborhood system of $a$.
Proof
Let $N$ be a neighborhood of $a$ in $M$.
Then by definition:
- $\exists \epsilon \in \R_{>0}: \map {B_\epsilon} a \subseteq N$
where $\map {B_\epsilon} a$ is the open $\epsilon$-ball at $a$.
The result follows from Open Ball in Real Number Line is Open Interval.
$\blacksquare$
Sources
- 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $2$: Metric Spaces: $\S 4$: Open Balls and Neighborhoods