# Algebra-I, Fall Solutions to Midterm #1

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1 Algebra-I, Fall Solutions to Midterm #1 1. Let G be a group, H, K subgroups of G and a, b G. (a) (6 pts) Suppose that ah = bk. Prove that H = K. Solution: (a) Multiplying both sides by b 1 on the left, we get b 1 ah = K. It follows that K is one of the left cosets of H. Since 1 K and the only left coset of H containing 1 is H itself, we conclude that K = H. Another solution: Taking inverses of both sides of the equality ah = bk, we get K 1 b 1 = H 1 a 1. Since H and K are subgroups, we have K 1 = K and H 1 = H, and therefore Ha 1 = Kb 1. Now multiplying this by the original equality on the right, we get H H = K K. Since H and K are subgroups, we get H = K. (b) (4 pts) Suppose that ah = Kb. Prove that H and K are conjugate. Solution: Let K = b 1 Kb (note that K is a subgroup). Then bk = Kb = ah. Thus K = H by (a), so H and K are conjugate. 2. Let G be a group and H a subgroup of G. Recall that H is called characteristic in G if ϕ(h) = H for any ϕ Aut(G). The subgroup H is said to be fully characteristic in G if ϕ(h) H for any ϕ End(G) where End(G) is the set of all homomorphisms from G to G. (a) (6 pts) Suppose that K is a subgroup of H and H is a subgroup of G. (i) Prove that if H is normal in G and K is characteristic in H, then K is normal in G. (ii) Give an example showing that if H is characteristic in G and K is normal in H, then K may not be normal in G. Solution: (i) Since H is normal in G, for any g G, the map ι g given by ι g (x) = g 1 xg is an automorphism of H. Since K is characteristic in H, we conclude that ι g (K) = K, which means that K is normal in G. (ii) Let G = A 4 and H = V 4 = {e, (12)(34), (13)(24), (14)(24)} and K = {e, (12)(34)}. Then H is characteristic in G since it is the only subgroup of G of order 4 (this holds, e.g. since all elements of G \ H have order 3). We also know that K is normal in H since H is abelian. On the other hand, K is not normal in G, e.g. since (123) (12)(34) (123) 1 = (23)(14) K. 1

2 (b) (4 pts) Give an example of a group G and a subgroup H of G which is characteristic, but not fully characteristic in G. Solution: We shall give two examples: Example 1: G = D 8 and H = r, the rotation subgroup. Then H is characteristic in G since it is the only subgroup of G isomorphic to Z 4. To see that it is not fully characteristic, take any s G \ H and K = r, s 2. Then K is a subgroup of index 2 in G (hence normal) and the quotient G/K is cyclic of order 2. Since the subgroup s of G is also cyclic of order 2, composing the natural projection from G to G/K with an isomorphism G/K s, we obtain a homomorphism ϕ : G G with Ker ϕ = K and Im ϕ = s = {1, s}. Since r K, we have ϕ(r) = s, so ϕ(h) is not contained in H and thus H is not fully characteristic. Example 2: Let A and B be any groups such that (1) A has is abelian, (2) B has trivial center (3) there is a non-trivial homomorphism ϕ : A B (For instance, we could take A = Z 2, B = S 3 and ϕ the map which sends the non-trivial element of Z 2 to (1, 2)). We claim that if we set G = A B and H = A {1 B }, then H is characteristic but not fully characteristic. It is easy to show that for any groups U and V the center of their direct product is equal to the direct product of the centers: Z(U V ) = Z(U) Z(V ). Thus, in our case Z(G) = Z(A) Z(B) = A {1 B } = H. It is also easy to see that the center is always characteristic, so H is characteristic in G. On the other hand, the map f : G G given by f((a, b)) = (1 A, ϕ(a)) is a homomorphism (where ϕ comes from (3) above), and f(h) contains an element of the form (1 A, b) with b 1 B, so H is not fully characteristic. 3. Let G be a group. (a) (5 pts) Suppose that H is a cyclic normal subgroup of G. Prove that H commutes with [G, G] elementwise, that is, hx = xh for any h H and x [G, G]. Solution: (a) Let ι : G Aut(H) be the conjugation homomorphism, that is, ι(g) = (h ghg 1 ) h H. Proving that hx = xh for any h H and x [G, G] is equivalent to showing that for any x [G, G] we have ι(x) = 1 Aut(H), that is, ι(x) is the identity element of the group Aut(H). If H is finite cyclic, then H = Z n for some n, whence Aut(H) = Aut(Z n ) = Z n. If H is infinite cyclic, then H = Z, whence Aut(H) = Z = Z2. In either case, Aut(H) is an abelian group, whence ι(g) is also abelian, being a subgroup of Aut(H). Since ι(g) = G/Ker ι, we conclude that Ker ι contains 2

3 [G, G], which means that ι(x) = 1 Aut(H) for x [G, G]. Another solution (sketch): Take any x, y G. Then our knowledge of Aut(H) yields that there exist n, m Z such that xhx 1 = h n and xhx 1 = h m for any h H. An explicit calculation shows that [x, y] 1 h[x, y] = x 1 y 1 xyhx 1 y 1 = h. Thus, h commutes with every commutator [x, y], whence h commutes with [G, G], the subgroup generated by all commutators. (b) (5 pts) Let H 1, H 2, H 3 be normal subgroups of G such that H i H j = G and H i H j = {1} for any pair of indices i j. Prove that subgroups H 1, H 2, H 3 are isomorphic to each other. Solution: By the diamond isomorphism theorem, for any i j we have H i = Hi /H i H j = Hi H j /H j = G/H j. In particular, H 1 = G/H3 and H 2 = G/H 3, so H 1 and H 2 are isomorphic. Similarly, H 1 and H 3 are isomorphic. 4. Let G be a finite group and H a normal subgroup of G. Let K be a G-conjugacy classes (that is, a conjugacy class of G) which is contained in H. Note that every H-conjugacy class is either contained in K or disjoint from K, so K is a union of k (distinct) H-conjugacy classes for some k. (a) (5 pts) Prove that k = [G : H C G (x)] where x is an arbitrary element of K. Solution: Let Ω be the set of H-conjugacy classes contained in K. We claim that G acts on Ω by conjugation to prove this we just need to show that conjugation by any g G sends an H-conjugacy class to an H-conjugacy class. In other words, we need to show the following: Claim: Let a, b G and g G. Then a and b are H-conjugate gag 1 and gbg 1 are H-conjugate. Proof of the Claim: We will show the forward implication ( ); the other direction is analogous. Suppose that a and b are H-conjugate, that is, b = hah 1 for some h H. Then gbg 1 = ghah 1 g 1 = (ghg 1 )(gag 1 )(ghg 1 ) 1. Since H is normal in G, we have ghg 1 H, whence gag 1 and gbg 1 are H-conjugate. Now we proceed with Problem 4(a). Fix x K, and let X be the H- conjugacy class of x. By the orbit-stablizer formula applied to the action of G on Ω constructed above we have Orbit(X) = [G : Stab G (X)]. Since this action is clearly transitive, we have Orbit(X) = Ω = k, so to prove the desired formula for k it is sufficient to show that Stab G (X) = H C G (x). Since any two H-conjugacy classes either coincide or intersect trivially, we have Stab G (X) = {g G : gxg 1 X} = {g G : gxg 1 = hxh 1 for some h H}. Since gxg 1 = hxh 1 h 1 g C G (x) g hc G (x) for some h H, we conclude that Stab G (X) = H C G (x). Another solution: As in the first solution, let X be the H-conjugacy 3

4 class of x. By the orbit-stabilizer formula applied to the conjugation action of H on G we have X = [H : C H (x)], and clearly C H (x) = C G (x) H. Since H is normal in G, the diamond isomorphism theorem yields [C G (x) : C G (x) H] = [HC G (x) : H], H which implies that [H : C H (x)] = C G = HC G(x) (x) H C G. On the other hand, (x) by the orbit-stabilizer formula applied to the conjugation action of G on itself we have C G (x) = G / K, and thus we conclude that X = HC G(x) C G (x) = HC G(x) K G = K [G : HC G (x)], whence [G : HC G (x)] = K. X As shown in the first solution, all H-conjugacy classes contained in K are of the same size (since they are G-conjugate to each other), and there are k of them by definition of k. Thus means that X = K k, whence k = K X = [G : HC G (x)]. Common gap in the exam papers: In many solutions using the second approach it was assumed without proof that all H-conjugacy classes inside K have the same size. (b) (2 pts) Let G = S n and H = A n, and pick some element σ K. Prove that k = 1 if C G (σ) contains an odd permutation and k = 2 otherwise. Solution: If C Sn (σ) contains no odd permutations, then C Sn (σ) A n, whence C Sn (σ)a n = A n. Thus, by part (a), k = [S n : C Sn (σ)a n ] = [S n : A n ] = 2. If C Sn (σ) contains an odd permutations, then C Sn (σ)a n is strictly larger than A n, and therefore C Sn (σ)a n = S n since A n already has index 2 in S n. Thus, k = [S n : S n ] = 1. (c) (3 pts) Once again, let G = S n and H = A n, let m n be an odd number and K the G-conjugacy class consisting of all m-cycles (note that K A n ). Prove that k = 2 if n m 1 and k = 1 otherwise. Solution: If σ is an m-cycle, then the conjugacy class of σ in S n consists of all m-cycles, and the number of distinct m-cylces is easily seen to equal n(n 1)...(n m+1) n! m. Thus, C m Sn (σ) = = m (n m)!. n(n 1)...(n m+1) If m = n or m = n 1, we conclude that C Sn (σ) = m, whence C Sn (σ) = σ since σ = m and σ C Sn (σ). Since σ A n, we have k = 2 by part (b). If m n 2, there is at least one transposition disjoint from σ. Such transposition is an odd permutation inside C Sn (σ), and therefore k = 1 by part (b). 5. Let G be a group of order 105 =

5 (a) (3 pts) Prove that G has a normal Sylow 5-subgroup OR a normal Sylow 7-subgroup. Solution: Let n p be the number of Sylow p-subgroups in G for p = 3, 5, 7. Standard application of Sylow theorems yields n 5 = 1 or 21 and n 7 = 1 or 15. If n 5 = 1 or n 7 = 1, then G has a normal Sylow 5-subgroup or a normal Sylow 7-subgroup. The only other possibility is that n 5 = 21 and n 7 = 15. Since distinct Sylow 5-subgroups and distinct Sylow 7-subgroups of G interesect trivially, in this case G will have (5 1) 21 = 84 elements of order 5 and (7 1) 15 = 90 elements of order 7. Since > 105 = G, we reach a contradiction. (b) (4 pts) Use (a) to prove that G has a normal subgroup of order 35. Deduce that G has a normal Sylow 5-subgroup AND a normal Sylow 7-subgroup. Solution: Let P 5 and P 7 be Sylow 5-subgroup and Sylow 7-subgroup of G, respectively. We know that at least one of the subgroups P 5 or P 7 is normal, so P 5 P 7 must be a subgroup of G. Furthermore, P 5 P 7 = P 5 P 7 / P 5 P 7 = 35. Since [G : P 5 P 7 ] = 3, by the small index lemma G has a normal subgroup H such that H P 5 P 7 and [G : H] divides 3! = 6. Since [G : H] [G : P 5 P 7 ] = 3 and [G : H] divides 105, the only possibility is that [G : H] = 3, so H = P 5 P 7 is a normal subgroup of order 35. Furthermore, by classification of groups of order pq given in class, we know that H = Z 5 Z 7 = Z35 since 5 (7 1). Now we shall prove that both P 5 and P 7 are normal. Take any g G. Since P 5 H and H is normal in G, we have gp 5 g 1 H. Thus, P 5 and gp 5 g 1 are both Sylow 5-subgroups of H. On the other hand, being abelian H has just one Sylow 5-subgroup. Therefore, gp 5 g 1 = P 5, so P 5 is normal in G. Similarly, P 7 is normal. (c) (3 pts) Prove that there are two isomorphism classes of groups of order 105 and describe them (briefly) using semi-direct products. You may use without proof that Aut(Z n Z m ) = Aut(Z n ) Aut(Z m ) if gcd(m, n) = 1. Solution: Let H be as in part (b) and let P 3 be a Sylow 3-subgroup of G. Since H is normal in G, H P 3 = G and gcd( H, P 3 ) = 1, we have G = H P 3. Since H = Z 35 and P = Z 3, the group G is isomorphic to the semi-direct product Z 35 ϕ Z 3 for some homomorphism ϕ : Z 3 Aut(Z 35 ). Non-trivial homomorphisms ϕ : Z 3 Aut(Z 35 ) correspond bijectively to elements of order 3 in Aut(Z 35 ). Since Aut(Z 35 ) = Aut(Z 5 Z 7 ) = Z 4 Z 6, it has exactly two elements of order 3, namely (0, 2) and (0, 4). Thus, there are exactly two non-trivial homomorphisms from Z 3 to Aut(Z 35 ), call them ϕ 1 and ϕ 2. The group Z 3 has an automorphism θ given by θ(x) = x, and it is clear 5

6 that ϕ 2 = ϕ 1 θ. Hence, by Observation 10.1 from class (=Problem 2(a) in HW#5), the semidirect products Z 35 ϕ1 Z 3 and Z 35 ϕ2 Z 3 are isomorphic. Thus, there are (at most) two possible isomorphism classes for G: the direct product Z 35 Z 3 = Z105 (corresponding to the trivial ϕ) and Z 35 ϕ1 Z 3 = Z105. Clearly, these two groups are non-isomorphic because the first one is abelian and the second one is not. 6

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