Let A and B be groups, and consider the product group G=A x
B.
(a) Prove that N={(ea,b) E A x B| b E B} is a
subgroup.
(b) Prove that N is isomorphic to B
(c) Prove that N is a normal subgroup of G
(d) Prove that G|N is isomorphic to A
Let G = Z x Z and H = {(a, b) in Z x Z | 8 divides a+b}
a. Prove directly that H is a normal subgroup in G (use the fact
that closed under composition and inverses)
b. Prove that G/H is isomorphic to Z8.
c. What is the index of [G : H]?
Prove Proposition 6.10 (Let f : X → Y and g : Y → Z be one
to one and onto functions. Then g ◦ f : X → Z is one to one and
onto; and (g ◦ f)−1 = f−1 ◦ g−1
).
Let A = Z and let a, b ∈ A. Prove if the following binary
operations are (i) commutative, (2) if they are associative and (3)
if they have an identity (if the operations has an identity, give
the identity or show that the operation has no identity).
(a) (3 points) f(a, b) = a + b + 1
(b) (3 points) f(a, b) = a(b + 1)
(c) (3 points) f(a, b) = x2 + xy + y2
Let (G,+) be an abelian group and U a subgroup of G. Prove that
G is the direct product of U and V (where V a subgroup of G) if
only if there is a homomorphism f : G → U with f|U =
IdU
Let f: X→Y and g: Y→Z be both onto. Prove that g◦f is an onto
function
Let f: X→Y and g: Y→Z be both onto. Prove that f◦g is an onto
function
Let f: X→Y and g: Y→Z be both one to one. Prove that g◦f is an
one to one function
Let f: X→Y and g: Y→Z be both one to one. Prove that f◦g is an
one to one function
Let G be a Group. The center of, denoted by Z(G), is defined to
be the set of all elements of G that with every element of G.
Symbolically, we have
Z(G) = {x in G | ax=xa for all a in G}.
(a) Prove that Z(G) is a subgroup of G.
(b) Prove that Z(G) is an Abelian group.
Let G be a cyclic group generated by an element a.
a) Prove that if an = e for some n ∈ Z, then G is
finite.
b) Prove that if G is an infinite cyclic group then it contains
no nontrivial finite subgroups. (Hint: use part (a))