Group
G , subgroup
H ⊆
G.
Can that inclusion be an epimorphism?
H of index 1 in
G ? — Yes (
H =
G).
H of index 2 ? — No (
H is normal in
G).
H of index ≥ 3 ? — Fix two cosets
Ha,
Hb of
H, distinct from each other and from
H ,
and define τ ∈ |
G|
! — an involution —
by giving τ(
x), for
x ∈
G, as
| |
|
{ |
xa−1b (= h0b ∈ Hb) , |
if x = h0a ∈ Ha ; |
| τ(x) | = | x ,
|
if x ∉ Ha ∪ Hb |
| | |
xb−1a ( = h0a), |
if x = h0b ∈ Hb . |
