Rationals are Everywhere Dense in Reals/Topology

Theorem

Then $\Q$ is everywhere dense in $\struct {\R, \tau_d}$.

Let $x \in \R$.

Let $U \subseteq \R$ be an open set of $\struct {\R, \tau_d}$ such that $x \in U$.

From Basis for Euclidean Topology on Real Number Line, there exists an open interval $V = \openint {x - \epsilon} {x + \epsilon} \subseteq U$ for some $\epsilon > 0$ such that $x \in V$.

Now consider the open interval $\openint x {x + \epsilon} \subseteq V$.

By Subset Relation is Transitive it follows that $\openint x {x + \epsilon} \subseteq U$.

Note that $x \notin \openint x {x + \epsilon}$.

From Between two Real Numbers exists Rational Number, there exists $y \in \Q: y \in \openint x {x + \epsilon}$.

As $x \notin \openint x {x + \epsilon}$, it must be the case that $x \ne y$.

That is, $V$ is an open set of $\struct {\R, \tau_d}$ containing $x$ which also contains an element of $\Q$ other than $x$.

As $V$ is arbitrary, it follows that every open set of $\struct {\R, \tau_d}$ containing $x$ also contains an element of $\Q$ other than $x$.

That is, $x$ is by definition a limit point of $\Q$.

As $x$ is arbitrary, it follows that all elements of $\R$ are limit points of $\Q$.

The result follows from the definition of everywhere dense.

$\blacksquare$