MAT301 Practice problems for Term Test 2
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MAT301 Practice problems for Term Test 2
(1) Prove that C* /R* has infinite order.
(2) Let G = (R* , .).
Find all subgroups of G of index 2.
(3) Let n ~ 3 and H a Sn be a normal subgroup such that (12) ∈ H . Prove that H = Sn .
Hint: Use that every element of Sn can be written as a product of transpositions.
(4) Let G = (Z, +). Let H = (5) and K = (7) . (a) Prove that G = HK
(b) Is G the internal direct product of H and K?
(5) Let G be a non abelian group of order 125 such that lZ(G)l > 1. Prove that G/Z(G) Z5 > Z5 .
(6) Let H, K - G be subgroups. Suppose H a G, k a G. Prove that HK is a normal subgroup of G.
(7) Let m, n > 1 be relatively prime. Let ϕ : Zm > Zn o Zm > Zn be an automorphism.
Prove that there are automorphisms ϕ 1 ∈ Aut(Zm) and ϕ2 ∈ Aut(Zn) such that ϕ(, ) = (ϕ1 (), ϕ2 ()).
Hint: Use that automorphisms preserve orders of elements.
(8) Let G be a group of order 30. Suppose Z(G) = {e}.
Prove that G can not be represented as the internal direct product of two nontrivial subgroups.
Hint: First show that if G = H x K and both lHl > 1 and lKl > 1 then lHl or lKl is prime.
(9) Suppose G1 > G2 > . . . > Gn is cyclic. Prove that each Gi is cyclic.
(10) Let ϕ : Z30 o Z6 x Z5 be an isomorphism such that ϕ() = (, ). Find all the possibilities for for ϕ(). Justify your answer.
(11) Determine the order of (Z > Z)/((2, 2)) . Is this group cyclic?
(12) Prove that (1 3 5) belongs to [A5, A5].
(13) For each of the following decide if H is not a subgroup, a subgroup which is not normal or a normal subgroup in G
(a) G = (C* , .), H = {z ∈ C* l lzl ∈ Q* }.
(b) G = D6 and H = {g ∈ D6 l g3 = e}.
(c) G = GL(2, R), H = {A ∈ GL(2, R) l lAl < e}.
(d) G = S3 > S3, H = {(g, g) lg ∈ S3}.
2022-11-20