How determinate the torsion subgroup and the normal maximal $\pi$-subgroup of $\mathbb{R}/\mathbb{Z}$?
57227
Maximal normal $\pi$-subgroups and torsion subgroups
156941
Subgroups of $\mathbb{Z}$
47731
Intersection of all $p$-Sylow subgroups is normal
12589
Show that every subgroup of the quaternion group Q is a normal subgroup of Q
30793
Intuition behind normal subgroups
141265
Finding a normal and not normal subgroup of $S_3$
64408
Subgroup of $\mathbb{Q}$ with finite index
67768
How to determine the group structure of $E(\mathbb{R})$ for an elliptic curve $E/\mathbb{R}$
2254
Finite group with isomorphic normal subgroups and non-isomorphic quotients?
101860
Relating the normal bundle and trivial bundles of $S^n$ to the tautological and trivial line bundles of $\mathbb{R}P^n$
61064
Finite index subgroup $G$ of $\mathbb{Z}_p$ is open.
144582
Index of a subgroup in $SL_2(\mathbb{Z})$
19720
A normal subgroup intersects the center of the $p$-group nontrivially
129943
Torsion on $\pi_1(X)$, $X$ connected and open in $\mathbb{R}^n$
2235
Showing $\text{Aut}_\mathsf{Grp}(\mathbb{Z}/2\mathbb{Z} \times \mathbb{Z}/2\mathbb{Z}) \cong S_3$
143314
Sylow subgroup of $S_{p^2}$
54911
The preimage of a normal subgroup under a group homomorphism is normal