$\mathrm{O}_n(\mathbb{Q})$ is dense in $\mathrm{O}_n(\mathbb{R})$
59477
Density of orthogonal matrices with rational coefficients
38507
Show that $\mathbb{Q}\times \mathbb{Q}$ is denumerable
157597
Homeomorphism between $\mathbb{Q}$ and $\mathbb{Q}(>0)$, and $\mathbb{Q}(\ge 0)$
4381
Proof that the set of irrational numbers is dense in reals
6161
The cartesian product $\mathbb{N} \times \mathbb{N}$ is countable
36747
$\text{Tor}_0^R(M,N)\simeq M\otimes_R N$
12212
Showing a certain subspace of Hilbert space is dense
50934
Let $x_n \in[0,1)$ s.t $x_n - \sqrt{n}\in \mathbb{Z}$. Prove that $X=\{x_n | n\in \mathbb{N}\}$ is dense in $[0,1]$.
36546
For an irrational number $a$ the fractional part of $na$ for $n\in\mathbb N$ is dense in $[0,1]$
95294
Dense subset of $C[0,1]$ which preserves rationals
27188
Bijection between $\mathbb{R}$ and $\mathbb{R}/\mathbb{Q}$
26256
Characterizing Dense Subgroups of the Reals
154754
Is $O_n$ isomorphic to $SO_n \times \{\pm I\}$?
78915
Density of the set $S=\{m/2^n| n\in\mathbb{N}, m\in\mathbb{Z}\}$ on $\mathbb{R}$?
120522
Showing any countable, dense, linear ordering is isomorphic to a subset of $\mathbb{Q}$
27670
Group homomorphisms $\mathbb{Q}\to\mathbb{R}$
132382
Showing a set is nowhere dense in $C([0,1])$