Is $\mathbb R-\mathbb Q$ open or closed?
105445
Are the rationals a closed or open set in $\mathbb{R}$?
64502
On the meaning of being algebraically closed
59424
Showing that $B=\{(x,y) \in \Bbb{R^2} \mid \ xy>0 \}$ is an open set
67259
Can we prove that a bounded closed subset of $\mathbb R^n$ is compact without Axiom of Choice?
3296
Proof That $\mathbb{R} \setminus \mathbb{Q}$ Is Not an $F_{\sigma}$ Set
127900
Interior of closure of open set in topological space
156101
Is the convex hull of closed set in $\mathbb R^{n}$ is closed?
80202
Why is $[0, 1] \cap \mathbb{Q}$ not compact in $\mathbb{Q}$?
73130
A simple question about open set
94825
Open set as a finite union of closed sets
27188
Bijection between $\mathbb{R}$ and $\mathbb{R}/\mathbb{Q}$
131082
Proof that $\mathbb R/[0,1]$ and $\mathbb R$ are homeomorphic
54406
Is $\mathbb Q \times \mathbb Q $ a denumerable set?
81927
Example of closed and bounded in $\mathbb Q$ set that is not compact
108955
Showing $(\mathbb{Q},+)$ is not isomorphic to $(\mathbb{R},+)$
10477
Finding the closure of A=$\mathbb Q \cap [0,1]$.
149317
An open subset of $\mathbb{R}$ is an at-most-countable union of disjoint open intervals