Math 417: Abstract Algebra Homework 7
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Homework 7
Math 417: Abstract Algebra
(Exercises are taken from Algebra: Abstract and Concrete, Edition 2.6 by Frederick M. Goodman.)
1. Write Z = {cI | c E R × }for the subgroup of non-zero multiples of the identity matrix in GLn(R). Show the following.
a. If n is odd, then SLn(R)Z = GLn(R) and SLn(R) u Z = {I}, and therefore PGLn(R) 六 SLn(R)
b. If n is even, then SLn(R)Z = GLn(+)(R), the subgroup of matrices A with det A > 0, and that
SLn(R) u Z = {干I}. Conclude that PGLn(R) contains an index 2 subgroup which is isomorphic to SLn(R)/{干I}.
2. Exercise §2.4.17 An automorphism of a group G is an isomorphism G 一 G from the group to itself. Fix g E G, and show that the function cg : G 一 G defined by cg(x) := gxg−1 is an automorphism of G
3. Exercise §2.5.13 The center of a group G is the set
Z(G) := {a ∈ G | ag = ga∀g ∈ G}
of elements which commute with every element of G. Show that Z(G) is a normal subgroup of G.
4. Exercise §2.7.6 Denote the set of all automorphism of G by Aut(G).
a. Show that Aut (G) is a group, with the operation of composition of functions.
b. Show that the function c : G → Aut(G) defined by g →7 cg is a homomorphism of groups.
c. Show that the kernel of c is Z(G).
d. The image of c is called the group of inner automorphisms of G and denoted Inn(G). Show that Inn(G) ≃ G/Z(G).
5. The purpose of this exercise is to compute the automorphism group of a finite cyclic group Let G = ⟨g⟩ be a finite cyclic group of order n.
a. Show that for any integer k ∈ Z, the function pk : G → G defined by pk(x) := xk defines a homomorphism from G to itself.
b. Show that pk is an isomorphism if and only if gcd(k, n) = 1.
c. Show that [k]n →' pk defines an isomorphism of groups Φ(n) → Aut(G).
6. Exercise §3.1.9 Show that the direct product A × B is abelian if and only if A and B are abelian.
7. Exercise §3.1.14 Show that Z4 × Z4 is not isomorphic to Z4 × Z2 × Z2. (See hint in book.)
8. Exercise §3.1.15 Let K1 be a normal subgroup of G1, and K2 a normal subgroup of G2 . Show that K1 × K2 is a normal subgroup of G1 × G2, and that (G1 × G2)/(K1 × K2) ≃ (G1/K1 ) × (G2/K2 )
2023-08-18