Rationals Dense in Reals

Theorem

Let $T = \left({\R, \tau}\right)$ be the Euclidean space of real numbers.

Let $x \in \R$.

Let $U \subseteq \R$ be an open set of $T$ such that $x \in U$.

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

Now consider the open interval $\left({x \,.\,.\, x + \epsilon}\right) \subseteq V$.

By Subsets Transitive it follows that $\left({x \,.\,.\, x + \epsilon}\right) \subseteq U$.

Note that $x \notin \left({x \,.\,.\, x + \epsilon}\right)$.

From Between Every Two Reals Exists a Rational, there exists $y \in \Q: y \in \left({x \,.\,.\, x + \epsilon}\right)$.

As $x \notin \left({x \,.\,.\, x + \epsilon}\right)$, it must be the case that $x \ne y$.

That is, $V$ is an open set of $T$ containing $x$ which also contains an element of $\Q$ other than $x$.

As $V$ is arbitrary, it follows that every open set of $T$ 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.

$\blacksquare$