Ifis a normal subgroup of
then the coset space
with the binary operation
for allbelonging to
is a group. The identity element of this group is the trivial coset
Let G be a group and let H be a subgroup of G. The following statements are equivalent:
-
is a normal subgroup of
-
For all
(The left and right cosets are identical)
-
Coset multiplication is well-defined that is if
and
then
Once this is established the rule for multiplication of cosets
follows.
Proof
If
is normal in
and and
then
so
Suppose
for all
Suppose
Then
Suppose coset multiplication is well defined. Let
so
and
so
is normal in
Now the main result: Ifis a normal subgroup of
the set of cosets
becomes a group under coset multiplication.
Proof
For associativity, note that
and
Hence,is the identity for coset multiplication.
Finallyso
for
and every coset (element of the set
has an inverse hence
is a group, called the quotient group of
by