MAT301 Week 5 Exercises
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MAT301 Week 5 Exercises
Exercise 1. Prove that the group of integers is isomorphic to all of its non-trivial subgroups.
Exercise 2. Let G be a group and let φ be an automorphism of G. Prove that the set of elements of G that are fixed by φ is a subgroup of G.
Exercise 3. For R = 勿, R, Q, C, what group is the multiplicative group {╱0(1) 1(a)、 : a e R} isomorphic to?
Exercise 4. Let G be an abelian group and let φ : G → G be the homomorphism defined by φ(g) = g2 .
1. Suppose that G is finite. Describe when φ is an automorphism of G.
2. Given an example of an infinite group G such that φ is injective, but not surjective.
Exercise 5. Let G be a group. For each g e G, let Inn(g) be the inner automorphism of G determined by g, that is, Inn(g) : G → G is defined by Inn(g)(x) = gxg - 1 .
1. Let g e G. When is Inn(g) = idG ? When is Inn(G) = {idG }?
2. Let g1 , g2 e G. When is Inn(g1 ) = Inn(g2 )?
3. Prove that the map Inn : G → Inn(G) is a homomorphism. What is its kernel?
Exercise 6. Let G and H be isomorphic groups and let φ, ψ : G → H be isomorphisms. Prove that {x e G : φ(x) = ψ(x)} is a subgroup of G.
Exercise 7. Prove that the dicyclic group Dic5 of order 20 is not isomorphic to the dihedral group D16 of order 20.
Exercise 8. Prove that R × is not isomorphic to the multiplicative group {╱0(a) b(0)、 : a, b e R × }.
Exercise 9. Find an injective homomorphism φ : Q>6 → Q>6 that is not surjective. Conclude that Q>6 is isomorphic to a proper subgroup of itself.
Exercise 10. Describe all homomorphisms from the additive group Q to itself. Conclude that there is no injective homomorphism from Q to itself that is not surjective, and therefore Q is not isomorphic to a proper subgroup of itself.
Exercise 11. Prove that Q × is not isomorphic to Q.
Exercise 12. Let φ e Aut(R × ). Prove that φ(R>6) = R>6 and φ(R<6) = R<6 .
Exercise 13. Let φ : G → H and ψ : H → K be homomorphisms. How are ker φ and ker(ψ О φ) related?
Exercise 14. Let φ : G → H be a surjective homomorphism. Prove that φ(Z(G)) ζ Z(H). Can you find a counterexample for this result if you drop the assumption that φ is surjective?
Exercise 15. Let G be a finite group. Prove that G is abelian if there exists an automorphism φ of G such that
1. φ2 = id; and
2. for all x e G, we have φ(x) = x if and only if x = e.
(Hint: Prove that if an automorphism φ of G with the above properties exists, then for all x e G there exists y e G such that x = φ(y)y - 1 , and think about what this tells you about φ(x). How can you prove that for all x e G there exists y e G such that x = φ(y)y - 1 without explicitly determining y? Make sure you use the hypothesis that G is finite!)
2022-06-14