# Problem 1.1. Classify all groups of order 385 up to isomorphism.

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1 Math 504: Modern Algebra, Fall Quarter 2017 Jarod Alper Midterm Solutions Problem 1.1. Classify all groups of order 385 up to isomorphism. Solution: Let G be a group of order 385. Factor 385 as 385 = Let n 5, n 7 and n 11 be the number of 5, 7 and 11-Sylow subgroups. Sylow s theorem implies that n 5 1(mod 5) and n This implies that n 5 = 1 or 11. Similarly, we have that n 7 1(mod 7) and n 7 55 which implies that n 7 = 1, and n 11 1(mod 11) and n which implies that n 11 = 1. In particular, we have a normal 7-Sylow subgroup N 7 G and a normal 11-Sylow subgroup N 11 G. Then N := N 7 N 11 G is a subgroup of order 77. Moreover, N is normal in G since for x N 7, y N 11 and g G, we have that g(xy)g 1 = (gxg 1 )(gyg 1 ) N 7 N 11 = N. Let K be any Sylow 5-subgroup. By considering orders of subgroups, we see that K N = 1 and KN = G. By the result from lecture, we know that G = N φ K where φ: K Aut(N) is a group homomorphism. We know that K = Z/5, N 7 = Z/7 and N11 = Z/11. Since N7 and N 11 are normal in G, they are also normal in N. As N 7 N 11 = 1, we know that N = Z/7 Z/11. We also know from lecture that Aut(N) = Aut(Z/7 Z/11) = Z/6 Z/10. We need to classify group homomorphisms φ: K = Z/5 Z/6 Z/10 = Aut(N). Since the element φ(1) Z/6 Z/10 has order dividing 5, we know that φ(1) = (0, 2i) for i {0, 1, 2, 3, 4}. There are two cases: Case 1: abelian. i = 0. In this case, G = N K = Z/77 Z/5 = Z/385 and is clearly Case 2: i 0. In this case, let α i : Z/5 Z/5 be the automorphism of groups defined by k ki. If φ i : Z/5 Z/6 Z/10 denotes the group homomorphism determined by φ i (1) = (0, 2i), then clearly φ i = φ 1 α i. By Homework Problem 3.1(1), we know that the two semi-direct products Z/77 φ1 Z/5 and Z/77 φi Z/5 are isomorphic. Therefore, we have only one non-trivial semi-direct product Z/77 φ1 Z/5 up to isomorphism. Since this group is not abelian, it is not isomorphic to Z/385 and we may conclude that there are only two groups of order 385 up to isomorphism: Z/385 and Z/77 φ1 Z/5. Problem 1.2. Let G = GL 3 (C) be the group of invertible 3 3 matrices with entries in C. Let H G be the subgroup consisting of diagonal matrices. (1) Find the centralizer C G (H) of H in G. (2) Find the normalizer N G (H) of H in G. (3) Determine the quotient N G (H)/H; that is, identify this group with a familiar group. 1

2 Solution: For (1), first observe that as H is abelian H C G (H). The centralizer C G (H) consists of 3 3 matrices a 1,1 a 1,2 a 1,3 A = a 2,1 a 2,2 a 2,3 a 3,1 a 3,2 a 3,3 such that AD = DA for every diagonal matrix d D = 0 d d 3. The condition that AD = DA translates into d 1 a 1,1 d 2 a 1,2 d 3 a 1,3 d 1 a 1,1 d 1 a 1,2 d 1 a 1,3 AD = d 1 a 2,1 d 2 a 2,2 d 3 a 2,3 = d 2 a 2,1 d 2 a 2,2 d 2 a 2,3 = DA. d 1 a 3,1 d 2 a 3,2 d 3 a 3,3 d 3 a 3,1 d 3 a 3,2 d 3 a 3,3 Since this must hold for every non-zero choice of d 1, d 2 and d 3, we conclude that a i,j = 0 if i j, that is, A H is diagonal. For (2), the normalizer N G (H) consists of 3 3 matrices A = (a i,j ) such that for every diagonal matrix D = diag(d 1, d 2, d 3 ) we have that DA = AD for another diagonal matrix D = diag(d 1, d 2, d 3). The condition that AD = D A translates into d 1 a 1,1 d 2 a 1,2 d 3 a 1,3 d 1a 1,1 d 1a 1,2 d 1a 1,3 AD = d 1 a 2,1 d 2 a 2,2 d 3 a 2,3 = d 2a 2,1 d 2a 2,2 d 2a 2,3 = D A. d 1 a 3,1 d 2 a 3,2 d 3 a 3,3 d 3a 3,1 d 3a 3,2 d 3a 3,3 Since this must hold for each d 1, d 2 and d 3, we see that A must have precisely one non-zero element in each row and each column. Indeed, if a i,j 0, then d j = d i. If a i,j 0 for j j, then d j = d i = d j which certainly doesn t hold for all d 1, d 2, d 3. Similarly, if i i, then d i = d j = d i which also can t hold. Let P 3 GL 3 (C) be the subgroup consisting of permutations matrices. Then P 3 H = where could be any non-zero complex number. We have verified that N G (H) P 3 H. It is a simple calculation to conclude that each of these matrices normalizes H, that is, P 3 H N G (H). We conclude that N G (H) = P 3 H. For (3), observe that P 3 H = 1. We also know that the subgroup P 3 of 3 3 permutation matrices is isomorphic to the symmetric group S 3. We may use the 2nd isomorphism theorem to conclude that N G (H)/H = P 3 H/H = P 3 /P 3 H = P 3 = S3. Alternatively, you can simply write down a surjective group homomorphism P 3 H P 3 which replaces an element in the normalizer with the matrix where each non-zero entry is replaced with a 1. The kernel of this homomorphism is clearly H. Therefore, N G (H)/H = P 3 H/H = P 3. 2

3 Problem 1.3. Find all possible composition series for the dihedral group D 12 of order 12. Solution: We write D 12 = r, s r 6 = s 2 = 1, rs = sr 1. We first observe that 1 r 3 r D 12 is a composition series with factors Z/3, Z/2 and Z/2. By the Jordan Hölder theorem, any composition series will have these factors up to reordering. Therefore, if 1 N 1 N 2 D 12 is a composition series, then N 2 is a normal subgroup of order 4 or 6. We claim that there are no normal subgroups of order 4. Since 12 = 2 2 3, a subgroup of order 4 is a 2-Sylow. The subgroup r 3, s is a 2-Sylow but it is not normal since rsr 1 = sr 4 / r 3, s. Since all 2-Sylow s are conjugate, there are no normal 2-Sylow subgroups. We now classify all subgroups of D 12 of order 6. Let N 2 D 12 be a subgroup of order 6, which is necessarily normal. By Cauchy s theorem, N 2 has a subgroup H N 2 of order 3. Then H D 12 is a 3-Sylow subgroup. Observe that r 2 D 12 is a 3-Sylow subgroup and that it is normal: indeed, sr 2 s = r 4 r 2. It follows that r 2 D 12 is the unique subgroup of order 3. Thus H = r 2 N 2 and N 2 is obtained from r 2 by adjoining an element of order 2. We can enumerate all elements of D 12 of order 2: s, sr, sr 2, sr 3, sr 4, sr 5, r 3. Adjoining these creates 3 distinct subgroups of order 6: r, r 2, s and r 2, sr. Observe that r = Z/6, which has 2 composition series: 1 r 3 r and 1 r 2 r. Also, both r 2, s and r 2, sr are isomorphic to S 3 which has a unique composition series 1 (123) S 3. Putting these observations together, we have four distinct composition series: 1 r 3 r D 12 1 r 2 r D 12 1 r 2 r 2, s D 12 1 r 2 r 2, sr D 12. Problem 1.4. Prove that a group of order p 2 q is solvable, where p and q are distinct primes. (Recall from HW Problem 2.2 that a finite group is solvable if and only if there exists a composition series with abelian factors.) Solution: Let G be a group of order p 2 q. Let n p and n q be the number of p and q-sylow subgroups, respectively. Sylow s theorem implies that n p 1(mod p) and n p q, and that n q 1(mod q) and n q p 2. Therefore, n p {1, q} and n q {1, p, p 2 }. If n p = 1, then there exists a normal subgroup N G of order p 2. From lecture, we know that N is isomorphic to Z/p 2 or Z/p Z/p and, in particular, N is abelian. In this case, G is solvable. If n q = 1, then there exists a normal subgroup N G of order q, and G/N is a group of order p 2, which (as above) is abelian as it is isomorphic to Z/p 2 or Z/p Z/p. In this case, G is again solvable. If n p = q and n q {p, p 2 }. Then q 1(mod p) so that q > p. Observe that n q p as this would imply that q < p. We are left with the case that n p = q and n q = p 2. In this case, we have p 2 Sylow subgroups of order q, and therefore p 2 (q 1) distinct elements of order q. There are only p 2 remaining elements in G which necessarily must form a p-sylow subgroup. Since all p-sylow s are conjugate, we see that this p-sylow is normal and that n p = 1, a contradiction. Problem 1.5. (1) Find a Sylow 7-subgroup of GL 3 (F 2 ). 3

4 (2) Show that GL 3 (F 2 ) is isomorphic to the subgroup of S 7 generated by the permutations ( ) and (15)(23). Hint: Consider the action of GL 3 (F 2 ) on the set F 3 2 \ 0 containing 7 elements and the induced homomorphisms GL 3 (F 2 ) S 7. Solution: Let us first observe that GL 3 (F 2 ) = (2 3 1)(2 3 2)( ) = 168 = by a simple counting argument: the first column has to be non-zero, the second column can t be a multiple of the first and the third column has to be linearly independent to the first 2. For (1), we simply want to find a matrix A GL 3 (F 2 ) of order 7 as this will generate a 7-Sylow. This can be found by trial and error. (In fact, it turns out that the number of 7-Sylows is 8 so there are 68 elements of order 7 so a random element has roughly a 28% chance of being an element of order 7.) Consider A = This element has order 7. One way to see this (other than laboriously computing the powers of A) is to think about how it acts on F 3 2 \ 0: A 0 A 0 A This shows that A has an element of order 7. (In fact, the way I found the matrix A was by playing around with the coefficients so that A would cycle through the 7 elements of F 3 2 \ 0.) 1 Order the elements of F 3 2 \ 0 by defining v i = A i For (2), the action of GL 3 (F 2 ) on the set F 3 2 \ 0 = {v 1,..., v 7 } induces a group homomorphism π : GL 3 (F 2 ) S 7 which is necessarily injective since a matrix that fixes every element of F 3 2 \ 0 must be the identity. Define the matrix B = so that B swaps v 1 and v 5, swaps v 2 and v 3, and fixes all other vectors. The matrix B was constructed so that π(b) = (15)(23) S 7. Since GL 3 (F 3 ) = im(π), it suffices to show that the matrices A and B generate GL 3 (F 3 ), or alternatively that the permutations ( ) and (15)(23) generate a subgroup of order 168. This can be proven in a variety of ways. The solution here is a variant of the clever solutions of Kristine Hampton and Thomas Lou. Let H = A, B GL 3 (F 3 ), then we only need to show H = 168 = We know H has an element of order 7 and 2. This implies that H has order 2 i 3 j 7 for i = 1, 2, 3 and j = 0, 1. Also A H is not normal since one can check that BAB 1 / A. This implies that the number n 7 of 7-Sylows in H is greater than 1. But in fact n 7 = 8 since n 7 = 1(mod 7) and n7 must divide 2 i 3 j, which in turn divides 24. Looking at the condition that 8 divides 2 i 3 j implies that i = 3 and j = 1, which shows that H =

5 (Alternatively, one could directly construct elements of order 3 (e.g., AB has order 3) and a subgroup of order 8. This requires playing around with elements a bit either in GL 3 (F 2 ) or S 7. ) Problem 1.6. Let k be a field. (1) Show that the ideal (xy zw) k[x, y, z, w] is prime. (2) Show that the element x k[x, y, z, w]/(xy zw) is irreducible but not prime. Solution: For (1), let R = k[y, z, w] and consider the degree 1 polynomial f = xy zw R[x]. The ideal p = (w) R is prime. The leading coefficient of f is y which is not in p and the constant coefficient is zw which is in p but not in p 2. By Eisenstein s criterion xy zw R[x] = k[x, y, z, w] is irreducible. Since k[x, y, z, w] is a UFD, it follows that (xy zw) is a prime ideal. For (2), we will use the following lemma: Lemma: Let f k[x 1,..., x n ] be an irreducible homogeneous polynomial in n variables and R = k[x 1,..., x n ]/(f). Then there is a function deg: R \ 0 Z 0 which is defined as follows: for g + (f) R, then deg(g + (f)) is defined as the smallest degree of a polynomial of the form g + fh for any h k[x 1,..., x n ]. For g 1, g 2 R \ 0, we have that deg(g 1 g 2 ) = deg(g 1 ) + deg(g 2 ). Proof of Lemma: Let S = k[x 1,..., x n ] and S d S be the homogeneous elements of degree d. Then S = d 0 S d as abelian groups with the property that for x S d and y S e non-zero, the product xy is in S d+e (in other words, S is a graded ring). Let π : S R be the canonical surjection onto the quotient. Define R d = π(s d ). Then it is easy to check that R = d 0 R d as abelian groups and that the multiplication of a non-zero element in R d by a non-zero element in R s is in R d+s. The degree (as we ve defined it above) of an element g = g g d R, with g i R i and g d 0, is precisely d. Finally, since f is an irreducible element, the ideal (f) is prime and therefore R is an integral domain. It follows that for non-zero elements g 1, g 2 R that deg(g 1 g 2 ) = deg(g 1 ) + deg(g 2 ). If it is possible to write x = q 1 q 2 with q 1, q 2 R = k[x, y, z, w]/(xy zw), then by the Lemma, we know that deg(q 1 ) + deg(q 2 ) = deg(x) = 1. Therefore deg(q 1 ) or deg(q 2 ) has to be zero in which case it is a non-zero constant and thus a unit. Therefore, x is irreducible. To see if x is prime, the quotient R/(x) = k[x, y, z, w]/(xy zw, x) = k[y, z, w]/(zw) which is not an integral domain. Therefore x R is not prime. 5

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