Is a discrete subgroup of a Hausdorff group closed?
111141
Why is every discrete subgroup of a Hausdorff group closed?
149980
Are there any countable Hausdorff connected spaces?
59654
Show that in a discrete metric space, every subset is both open and closed.
127173
Closed subset of compact set is compact
117000
Group presentation for discrete Heisenberg group
156941
Subgroups of $\mathbb{Z}$
48162
Is the subset $[0, \sqrt2] ∩\mathbb{Q} ⊂ \mathbb{Q}$ closed, bounded, compact?
147331
Compactness and Hausdorffness
2915
$X$ is Hausdorff if and only if the diagonal of $X\times X$ is closed
149220
If every compact set is closed, then is the space Hausdorff?
151130
When is the product of $n$ subgroups a subgroup?
141642
Closure of a certain subset in a compact topological group
26256
Characterizing Dense Subgroups of the Reals
33309
Subgroup of $\mathbb{R}$ either dense or has a least positive element?
54346
If $G$ is a compact topological group, how to show that a finite index subgroup of $G$ is open?
143586
Is $[0,1]$ a union of family of disjoint closed intervals?
70298
Are quotients of chain groups $C_n(X)/C_n(A)$ still free?