Open Reciprocal-N Balls form Neighborhood Basis in Real Number Line

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Theorem

Let $\R$ denote the real number line with the usual (Euclidean) metric.

Let $a \in \R$ be a point in $\R$.


Let $\BB_a$ be defined as:

$\BB_a := \set {\map {B_\epsilon} a: \epsilon \in \set {\dfrac 1 n: n \in \N} }$

that is, the set of all open $\epsilon$-balls of $a$ for $\epsilon$ which are reciprocals of integers.


Then $\BB_a$ 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 $a$.

From Open Ball in Real Number Line is Open Interval:

$\map {B_{\epsilon'} } a = \openint {a - \epsilon'} {a + \epsilon'}$

From Between two Real Numbers exists Rational Number:

$\exists \epsilon'' \in \Q: 0 < \epsilon'' < \epsilon'$

Let $\epsilon''$ be expressed in canonical form:

$\epsilon'' = \dfrac p q$

Let $\epsilon = \dfrac 1 q$

Then $\epsilon \le \epsilon'' < \epsilon'$

and so:

$\openint {a - \epsilon} {a + \epsilon} \subseteq \openint {a - \epsilon'} {a + \epsilon'}$

From Open Real Interval is Open Ball

$\map {B_\epsilon} a = \openint {a - \epsilon} {a + \epsilon}$

is the open $\epsilon$-ball at $a$.

From Subset Relation is Transitive:

$\openint {a - \epsilon} {a + \epsilon} \subseteq N$

From Open Ball is Neighborhood of all Points Inside, $\openint {a - \epsilon} {a + \epsilon}$ is a neighborhood of $a$ in $M$.


Hence there exists a neighborhood $\openint {a - \epsilon} {a + \epsilon}$ of $a$ which is a subset of $N$.

Hence the result by definition of basis for the neighborhood system of $a$.

$\blacksquare$


Sources