# Yale University Department of Mathematics Math 350 Introduction to Abstract Algebra Fall Midterm Exam Review Solutions

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1 Yale University Department of Mathematics Math 350 Introduction to Abstract Algebra Fall 2015 Midterm Exam Review Solutions Practice exam questions: 1. Let V 1 R 2 be the subset of all vectors whose slope is an integer. Let V 2 R 2 be the subset of all vectors whose slope is a rational number. Determine if V 1 and/or V 2 is a subgroup of R 2, with usual vector addition. Solution. V 1 contains zero (if one defines the slope of the origin to be 0, is closed under taking inverses (negation actually preserves slope, but is not closed under addition. For example, v = (1, 2 has slope 2 and w = (1, 1 has slope 1, but v + w = (3, 2 has slope 3/2. V 2 contains zero, is closed under taking inverses, but is not closed under addition. For example, v = (1, 0 has slope 0 and w = ( 2, 2 has slope 1, but v + w has slope 2/(1 + 2 = 2 2, which is not rational. 2. Write down a nontrivial homomorphism ϕ : Z/36Z Z/48Z and compute its image and kernel. Solution. Since the domain is a cyclic group, we only need to specify where a generator is sent, and verify the relations. So we need to choose ϕ(1 whose order divides 36. For example, gcd(36, 48 = 12, so we could choose ϕ(1 to be any element of order 12 in Z/48Z, for example, 48/12 = 4 has order 12. So the choice of φ(1 = 4 will produce a well defined (and nontrivial homomorphism ϕ : Z/36Z Z/48Z. The image is the cyclic subgroup 4 Z/48Z, which is itself a cyclic group of order 12. Since ϕ(1 has order 12, it shows that ϕ(12 = 0 and in fact that the ker(ϕ is the cyclic subgroup 12 Z/36/Z, which is itself a cyclic group of order 36/12 = 3. Of course, any choice of element of Z/48Z whose order divides 36 would have worked, for example, 24 Z/48Z has order 2, which gives another nontrivial example. 3. How many elements of order 6 are there in S 6? In A 6? Solution. Considering the disjoint cycle decomposition, and the formula for the order of a product of disjoint cycles as the lcm of the cycle lengths, the only elements of order 6 in S 6 are the 6-cycles or the (2, 3-cycles. There are 5! choices of 6-cycles, indeed, a 6-cycle must contain all numbers 1,..., 6 and we can always cyclically permute so that 1 is the first number, then there 5! distinct choices for the rest of the numbers. There are 2 (6 4 2( 3 choices of (2, 3-cycles, indeed, choosing a 2-cycle is equivalent to choosing 2 elements out of 6 and then 3 elements out of the remaining 4, with the understanding that for each choice there is a unique 2-cycle and two possible 3-cycles with those given sets of numbers. (Or you can memorize formulas in the book for the number of n-cycles in a symmetric group. In total, there are = 240 elements of order 6 in S 6 (which is 1/3 of the elements!.

2 4. Prove that is divisible by 17. Solution. We use Euler s theorem to compute mod 17. Since (mod 17 we reduce 104 = mod 16, so that (mod 17. Now 11 8 = ( = 121 8, so we can simplify by reducing 121 = mod 17, so that (mod 17. Then (mod 17, implying that is divisible by Let G be a finite group and x, y G be nonidentity elements such that xyx 1 = y 1. Show that G has an element of order 2. Find an infinite group G where this breaks down. Solution. If y has order 2 we are done, so we can assume that y > 2, in which case y y 1. Now consider the subgroup H G generated by x and y. The crucial observation is that H acts on the set A = {y, y 1 } by conjugation. Indeed, x acts by exchanging the elements of A and y does nothing, so any element in H either exchanges the elements of A or leaves them fixed. We get a permutation representation σ : H S A = Z/2Z, which is surjective, since x maps to the nonidentity element. By the 1st isomorphism theorem, we have that H = ker(σ Z/2Z is even. By Cauchy s theorem, H has an element of order 2, hence so does G. The second part is really hard (must harder than what you ll find on the midterm, but I wanted you to try to get it! To find an infinite group where the conclusion fails, build the universal group where xyx 1 = y 1 holds, namely, the group with presentation G = x, y xyx 1 = y 1. First we can argue that this group is nontrivial and that both x and y have infinite order. To see this, we can define a homomorphism G D 2n by specifying where the generators are sent: x s and y r. The relation is clearly satisfied by the usual presentation of the dihedral group, so this defines a homomorphism, which is clearly surjective. So G cannot be the trivial group and, in fact, G must be nonabelian (it maps surjectively to a nonabelian group. Also, neither x nor y can be the identity, and moreover, y has infinite order (since it map to elements of any given order, it cannot itself have finite order. We can also define a surjective homomorphism G Z Z/2Z by sending x (1, 0 and y (0, 1 and checking the relation. Thus shows that x also has infinite order. Next, considering the same conjugation action on the set {y, y 1 } as before, we get a surjective permutation representation homomorphism σ : G Z/2Z. One can check that K = ker(σ is generated by x 2 and y. One thing you might have noticed while playing around with the relation is that it implies that yx 2 = x 2 y. Thus K is generated by two commuting elements, i.e., has a presentation K = u, v uv = vu. Thus, in fact, K is isomorphic to Z Z. In particular, K has no elements of finite order. Now assume that G has some element z of order 2, then z K. As K G has index 2, we have a disjoint union G = K xk, hence z xk. Write z = xu for some u K. Then 1 = z 2 = xuxu = xux 1 x 2 u = xux 1 ux 2, since u and x 2 are in K, so commute. Note that, for a general u K, we have that xux 1 u is a power of x 4. Indeed, since we can write u = y n x 2m, we have that xux 1 = xy n x 1 x 2m = y n x 2m, so that xux 1 u = x 4m. However, assuming the existence of z, we got that xux 1 u = x 2, which is impossible. Hence there is no element z of order 2 in G. 6. Write down two elements of S 10 that generate a subgroup isomorphic to D 10. (Hint: Use the left multiplication action on D 10. Solution. If we order the elements of D 10 = {1, r,..., r 4, s, sr,..., sr 4 } in the usual way, then we can compute the permutations induced the elements of D 10 by left multiplying by

3 r and s. We see that r corresponds to the permutation (12345( and s corresponds to the permutation (16(27(38(49(5 10. Since the left multiplication action is always faithful, the image of its permutation representation is a subgroup of S 10 isomorphic to D 10 and generated by the images of r and s. 7. Consider the left regular permutation representation S n S n!. Describe the cycle type in S n! of the image of an n-cycle in S n. Solution. Let σ be an n-cycle and z any element of S n. Then the cycle containing z in the permutation induced by left multiplication by σ on S n, is just {z, σz, σ 2 z,..., σ n 1 z}. Indeed, if σ i z = σ j z, then i j (mod n. If we imagined ordering all n! elements of S n, then we see that σ would permute the elements as a disjoint product of n-cycles, in fact (n 1! of them. In fact, the same argument shows that if σ is any element of order k in S n, then the cycle type of the permutation induced by σ via left multiplication, is a product of n!/k disjoint k-cycles. This makes all permutation in S n look regular. 8. Prove that C Sn ( (12(34 has 8 (n 4! elements for n 4 and explicitly determine all of them. Solution. We know that the size of the conjugacy class in S n containing σ = (12(34 is [S n : C Sn ((12(34]. But we also know that this conjugacy class consists of all type (2, 2-cycles. We can count the number of them. Choosing a type (2, 2-cycle is equivalent to choosing 2 elements out of n and then 2 elements out of the remaining n 2, and remembering that we can switch the order of the two disjoint 2-cycles we ve just chosen. So the number is 2( 1 n ( n Thus n! C Sn ((12(34 = = 8 (n 4! 1 n ( n 2 2( 2 2 Explicitly, C Sn ((12(34 = C S4 ((12(34 S n 4, where S n 4 is the symmetric subgroup on {4, 5,..., n}, and C S4 ((12(34 = {1, (12, (34, (12(34, (13(24, (14(23, (1324, (1423}. 9. Show that the set of nonzero matrices of the form a 3b b a is a cyclic subgroup of GL 2 (F 5. What is the order of this subgroup? Solution. Let s denote this matrix by M(a, b. First, note that there are 24 such choices of nonzero matrices M(a, b, since each of a and b can range over F 5, but both can t be zero. Next, note that det M(a, b = a 2 3b 2 is only zero when a = b = 0, which we can check directly, noting that the only squares in F 5 are 0, 1, and 4. So these 24 matrices are certainly contained in GL 2 (F 5. We also see that M(a, bm(c, d = M(c, dm(a, b = M(ac + 3bd, ad + bc, hence this subset is closed under multiplication and all elements commute, so it forms an abelian subgroup of GL 2 (F 5. To prove that it is cyclic, we need to show that (at least one of these elements has order 24. We first note that 2 and 3 have order 4 in F 5, so M(2, 0 = M(3, 0 = 4. Next, let s look at the next easiest case, M(0, a 2 = M(3a 2, 0, hence M(0, b = 8 for any b F 5, in view of the fact that 3a 2 is always either 3 or 2. Now, if we can also find an element of order 3, then its product with an element of order 8 will have order 24, by PS 1 (we are in

4 an abelian group. To find an element of order 3, we are looking for a matrix that satisfies the polynomial x 3 1 = (x 1(x 2 + x + 1. So if it satisfies x 2 + x + 1, then it will have order 3. The characteristic polynomial of M(a, b is x 2 2ax + a 2 3b 2, so that choosing (a, b = (2, 1, for example, gives a matrix M(2, 1 that satisfies the correct polynomial (by the Cayley Hamilton theorem so has order 3. Hence M(0, 1M(2, 1 = M(3, 2 has order 24, and we ve just proved that this subgroup is cyclic of order Find square matrices A and B with coefficients in the field F p such that AB BA = I. What size do the matrices need to be? (Note that this is impossible for real matrices because Tr(AB = Tr(BA! Solution. Over any scalar field, since Tr(AB = Tr(BA, we have that Tr(AB BA = 0. Hence if AB BA = I for some n n matrices, then Tr(I = n must be zero in the field. So, over a field of characteristic zero (i.e., where no integer is zero, no such matrices can exist. Over a field of characteristic p, in order to find such matrices, we must have p n. Playing around with matrices, one can, for p small, find explicit p p matrices that do the job. For example, for p = 2, A = 0 1, B = works. For p = 3, A = , B = works, and going on this way, one could start to see a pattern emerge. However, here is a really nice construction of such matrices. It is not easy to just come up with this out of nowhere, but it s nice so I wanted to show it to you. First, let F be any field, and let F [x] denote the (countably infinite dimensional vector space of polynomials p(x with coefficients in F. Define linear maps A : F [x] F [x] to be differentiation and B : F [x] F [x] to be multiplication by x. Then the product rule gives, for any p(x F [x], d dx (xp(x = x d p(x + p(x dx which we can rewrite as d dx (xp(x x d dxp(x = p(x, i.e., AB BA = id. This gives an infinite dimensional example over any field. We could even write the infinite by infinite matrix representations of these linear maps (with respect to the standard basis of F [x] to get explicit infinite matrices. Now let F = F p. Then something special happens, namely, d dx xp = 0. This gives a clue that we can truncate the infinite dimension example into a p-dimensional example. Let V be the F p -vector space of polynomials of degree at most p 1 over F p. Then V has dimension p with basis 1, x,..., x p 1. Let A be differentiation again and now let B be multiplication by x, but taken modulo x p (which later in the semester will actually have a legitimate meaning, by which we mean B(x i = x i+1 if i < p 1 and B(x p 1 = 0. Then actually A and B still work, and now they are linear operators on a p-dimensional vector space! By looking at the matrix representations of A and B (with respect to the standard basis, we can even get explicit p p matrices.

5 11. Find the highest power of p dividing the order of GL n (F p. Find a Sylow p-subgroup of GL n (F p. (Hint: Think upper triangular. Solution. From class, we ve seen several times that GL n (F p = (p n 1(p n p(p n p 2 (p n p n 1 we can factor (n 1 = n(n 1/2 powers of p out and what remains (p n 1(p n 1 1 (p 1 will not be divisible by p. Following the hint, and being inspired by some stuff we did form the last problem set, we can see that the subgroup (you basically checked that this was a subgroup in homework of all unipotent matrices, i.e., upper triangular matrices with ones on the diagonal, has n(n 1/2 spots where any element of F p can go, so the order of this subgroup is p n(n 1/2, hence it s a Sylow p-subgroup.

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