a. D6 = {1,
a,
a2,
a3,
a4,
a5,
b,
ba,
ba2,
ba3,
ba4,
ba5};
K =
Z(
D6) = {1,
a3} by Exercise 26 §2.6. The cosets are


We have

,

and
KaKbKa =
Kaba =
Kb. Hence
D6/
K ≅
D3.
c. G =
A ×
B,
K = {(
a, 1)
a
A}. Note
K G because (
x,
y)
−1(
a, 1)(
x,
y) = (
x−1ax, 1)
K. If
b
B,
a
A, note that (
ab)(1,
b)
−1
K, so

Thus G/K = {K(1, b) b B}. Observe that K(1, b)K(1, b′) = K(1, bb′) gives ...