Let H, K be groups and α : K → Aut(H) be a homomorphism of
groups....
Let H, K be groups and α : K → Aut(H) be a homomorphism of
groups. Show that H oα K is the internal semidirect product of
subgroups which are isomorphic to H and K, respectively
Let φ : G1 → G2 be a group homomorphism. (abstract algebra)
(a) Suppose H is a subgroup of G1. Define φ(H) = {φ(h) | h ∈ H}.
Prove that φ(H) is a subgroup of G2.
(b) Let ker(φ) = {g ∈ G1 | φ(g) = e2}. Prove that ker(φ) is a
subgroup of G1.
(c) Prove that φ is a group isomorphism if and only if ker(φ) =
{e1} and φ(G1) = G2.
Let f : G → G′ be a surjective homomorphism between two groups,
G and G′, and let N be a normal subgroup of G. Prove that f (N) is
a normal subgroup of G′.
Direct product of groups: Let (G, ∗G) and (H,
∗H) be groups, with identity elements eG and
eH, respectively. Let g be any element of G, and h any
element of H. (a) Show that the set G × H has a natural group
structure under the operation (∗G, ∗H). What
is the identity element of G × H with this structure? What is the
inverse of the element (g, h) ∈ G × H? (b) Show that the map...
Let G, H be groups and define the relation ∼= where G ∼= H if
there is an isomorphism ϕ : G → H.
(i) Show that the relation ∼= is an equivalence relation on the
set of all groups.
(ii) Give an example of two different groups that are
related.
Let M/F and K/F be Galois extensions with Galois groups G =
Gal(M/F) and H = Gal(K/F). Since M/F is Galois, and K/F is a field
extension, we have the composite extension field K M.
Show that σ → (σ|M , σ|K) is a
homomorphism from Gal(K M/F) to G × H, and that it is one-to-one.
[As in the notes, σ|X means the restriction of the map σ
to the subset X of its domain.]
Let H and K be subgroups of a group G so that for all h in H and
k in K there is a k' in K with hk = k'h. Proposition 2.3.2 shows
that HK is a group. Show that K is a normal subgroup of HK.
Let G be a group and K ⊂ G be a normal subgroup. Let H ⊂ G be a
subgroup of G such that K ⊂ H Suppose that H is also a normal
subgroup of G. (a) Show that H/K ⊂ G/K is a normal subgroup. (b)
Show that G/H is isomorphic to (G/K)/(H/K).