Proper subset of the set of irrationals such that it is countable and dense in $\Bbb R$ We know that $\Bbb R$ is separable i.e. it contains a dense subset which is countable. We have $\Bbb Q$ and ${\Bbb R} -  {\Bbb Q}$ to be dense subsets respectively countable and uncountable. I was looking for a countable dense subset of $\Bbb R$ which is a proper subset of either (i) $\Bbb Q$ or (ii) ${\Bbb R} -  {\Bbb Q}$ .
For (i), by considering the set of dyadic rationals i.e. $\{\frac{a}{2^b} | a \in \Bbb Z , b \in \Bbb N \}$ or more generally for any fixed prime $p \in \Bbb N$, consider, $\{\frac{a}{p^b} | a \in \Bbb Z , b \in \Bbb N \}$ .
It is a countable proper subset of $\Bbb Q$ which is dense.
But I could not come up with any example for (ii) . Thanks in advance for help.
 A: Choose any rational number $a$. Then $\mathbb{Q}-\{a\}$ is a proper, dense subset of $\mathbb{Q}$.

Choose any irrational number $b$. Then $\{a+b \mid a \in \mathbb{Q}\}$ is a countable, dense subset of $\mathbb{R}-\mathbb{Q}$.
A: $$
\left\{q\sqrt{2}: q\neq 0 \text{ rational }\right\}.
$$
A: You may also pick a countable dense set whose elements are all trascendental (and in such a way that this set is not the set of all rational multiples of some fixed transcendental, which would be the right boring answer). As algebraic numbers are countable, the set $T$ of all transcendentals is dense in $\mathbb{R}$. "Having a countable base" is a hereditary topological property, so the space $T$ of transcendentals has a countable base. Spaces with a countable base are separable, that is, have a countable dense subset (just pick one point in each basic open set). This means that $T$ contains a countable dense subset $D$. As $D$ is dense in $T$ and $T$ is dense in $\mathbb{R}$, $D$ is dense in $\mathbb{R}$. So, $D$ is a countable dense subset of the real line whose elements are all transcendental. From time to time I pose such question to my students...
