Number of index $2$ subgroups in $S_n$
1996
$A_n$ is the only subgroup of $S_n$ of index $2$.
84132
Number of preimages of a group element under a homomorphism.
12667
How many subgroups of order $n$ does $D_n$ have?
106660
Every subgroup of the integers has finite index
7108
Normal subgroup of prime index
58937
Finite Index of Subgroup of Subgroup
142038
Finding the number of elements of order two in the symmetric group $S_4$
99999
A question on identifying normal subgroups of $S_4$
155502
Index of a maximal subgroup in a finite group
115204
Show that SO(n) is a normal subgroup of O(n)
156154
Sylow 2 subgroups of S4
83061
Generating the symmetric group $S_n$
86336
For $G$ group and $H$ subgroup of finite index, prove that $N \subset H$ normal subgroup of $G$ of finite index exists
71245
$2 \times 2 $ Orthogonal Group
128834
Proving the number of even and odd permutations of a subgroup $H<S_{n}$ are equal, provided $H$ is not contained in $A_{n}$
14895
Reference for the subgroup structure of $\rm{PSL}_2(q)$
47824
Calculation of subgroups of $(Z_{12}, +)$