# Math 594, HW2 - Solutions

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1 Math 594, HW2 - Solutions Gilad Pagi, Feng Zhu February 8, a). It suffices to check that NA is closed under the group operation, and contains identities and inverses: NA is closed under the group operation since N is normal: nan a = n(an a 1 )aa = nn aa NA. e A N since both are subgroups, so e = ee NA. Given na NA, (a 1 n 1 a)a 1 NA since N is normal, and na(a 1 n 1 a)a 1 = e. b). Define a map N γ A NA by (n, a) na. This is a group map, since (n, a)(m, b) = (nγ a (m), ab) nama 1 ab = namb. Moreover this map is surjective by construction. The map is an isomorphism iff it is injective, iff na e for (n, a) (e, e), which happens iff N A = {e} (otherwise we can find N n e with n 1 A, and then (n, n 1 ) e.) c). N N φ A as n (n, e); A N φ A as a (e, a); (the isomorphic image of) N is normal in N φ A since (m, b)(n, e)(m, b) 1 = (m, b)(n, e)(b 1 m 1, b 1 ) = (mb n, b)(b 1 m 1, b 1 ) and N A = {e} by construction. = (mb nm 1, e) N d). Taking the last computation from the previous part and setting m = e (so that (m, b) = (e, b) A, we obtain (e, b)(n, e)(e, b) 1 = (b n, e) i.e. conjugation of N by an element a A is equivalent to the action of a on N, as desired. And the indirect help of Umang Varma, Lara Du 1

2 e). The key here is to prove that G = MN: N, M normal in G so MN = NM G. Since MN M, N it must be then G = MN or MN = M or MN = N otherwise,using the forth iso theorem, the image of MN under the quotient G G/M will give a non trivial subgroup. Same for N, But N M so it must be G = NM. Now the rest follows: Observe the canonical map MN = G G/M G/N. The kernel is M N so G/(M N) = G/M G/N. G/M = NM/M = N/(N M) by using the second iso theorem. And symmetrically for M. 2 a). Let ψ be the group map K Aut(H) described, and let r and x denote generators of H and K resp.; then, applying the result of 1(d) to write the action of K on H as conjugation by elements of K, H ψ K = r, x xrx 1 = r 1 = D n, where the isomorphism is given by sending r to a rotation through 2π, and x to any reflection. n b). Let E be the transformation inverting one coordinate (i.e. after fixing a basis, represented by a diagonal matrix with 1 on the diagonal in n 1 places and 1 at one place. Notice: Det(E) = 1, E / SO n (R), E 2 = I. Define an action {I, E} AutSO n (R) by E(A) = EAE 1 = EAE. Since SO n is of index 2, it is normal, then we get the semi-direct product group and it must be G = SO n (R) E and in fact G = SO n (R) E. Specifically, define SO n (R) E O n (R) by (A, B) AB, and observe that in fact this is an isomorphism. This is not a direct product because E is not normal in O n (R): EA is the A matrix with the first row multiplied by -1, whereas AE is the A matrix where the first column is multiplied by -1. c). Both O 2, R 2 E, intersection is trivial so it is a semi-product if one of the subgroups is normal. If T is translation by t, and A O 2 then AT A 1 is a translation by Atas seen in the computation below, thus R 2 E and we are done. The action O 2 Aut(R 2 ) must be conjugation in E, where the actual automorphism is quite natural: let t be translation so x x + At is the same as x A 1 x A 1 x + t A(A 1 x + t) = x + At 3 a). Given a split, we denote A as the image of A in G and denote B as the image of B in G under φ. WTS G = A B, A G, A B = e. First, consider g G, g / A. chasing the diagram g b e B g B b B. So under π, the images of g, g are the same, and the ker = A so g g 1 A B A. Indeed A G. Finally, chasing a member of the kernel with preimage in B must be e. Alternatively, consider G = N H. So the following is exact: 1 N G π H 1 using the projections, and N normal. Consider φ : a (e, a) so πφ = id H. 2

3 b). All the subgroups of Q are normal so every semi-product would be direct product, but Q is not abelian. 1 1 Q x x2 Klein 1. 4 a). Let H(k) denote the Heisenberg subgroup. Note H(k) GL 3 (k) as a subset. Now the identity is in H(k), since we may take a = b = c = 0, and that H(k) is closed under composition and under taking inverses: the composition of two elements is given by 1 a b 1 a b 1 a + a b + b + ac 0 1 c 0 1 c = 0 1 c + c and inverses may be obtained by taking a = a, c = c and b = ac b in the above. Hence H(k) is a subgroup of GL 3 (k), as claimed. b). We certainly have H H(k) as a subset, and H contains the identity; moreover, from the above computation, H is closed under composition (a = a = 0 = a + a = 0) and inverses (a = 0 = a = 0), so it is a subgroup of H(k). We may verify that it is isomorphic to k 2 by the isomorphism 1 0 b b, 0 1 c c H is a normal subgroup of H(k), since it is the kernel of the group map H(k) k given 1 a b by 0 1 c a c). {id} K H(k) in the category of sets; from the above computation, K is closed under composition (b = b = c = c = 0 = b + b + ac = c + c = 0) and inverses (b = c = 0 = c = ac b = 0), so it is a subgroup of H(k). We may verify that it is isomorphic to k by the isomorphism K is a not a normal subgroup of H(k): a for a 0. 3 = 1 a a a a. / K

4 d). Denote G as the Heisenberg group. Notice that together they generate G: 1 a b ac 1 a b c = 0 1 c So G = HK, H( G, ) H ( K = I so ) we have direct product. The action must be by x x + ya conjugation a =. y y e). F 2 = C2, F 2 2 = Group of order 4, So G is of order 8. G is not abelian so either D 4 or Q. Since F 2 2 already contains 3 elements of order 2, this can t be Q. f). H(F p ) 5 a). D q = 2q and we have a 2-Sylow of the reflection t, and the rotation r is a q-sylow. b). We may characterize GL n (F p ) as the group of n n matrices of full rank over F p. We may count these as follows: there are p n 1 choices for the first column (it can be any nonzero column), p n p choices for the second column (it can be anything which is not a multiple of the first column), p n p 2 choices for the third column (anything not in the span of the first two columns), etc., up to p n p n 1 choices for the last column; hence GL n (F p ) = (p n 1)(p n p) (p n p n 1 ) and in particular p 1+ +(n 1) = p (n 2) divides GLn (F p ) and so we will be done if we can show that the subgroup of upper triangular matrices with ones on the diagonal has cardinality ( p (n 2) ; but this is clear since there are p choices for each of the (n 1) = n ) 2 matrix entries above the diagonal (note that all upper triangular matrices with ones on the diagonal have determinant one and are already invertible, and we have no further restrictions.) 6 a). By Lagrange s theorem, p t n G ; in particular p t P. Let P = p t+k where k 0. Observe G = P Y orbit Y, where we sum over the orbits when H acts on G/P. If every orbit has cardinality divisible by p, then G is divisible by P p = p t+k+1, which contradicts that P is a p-sylow group of G. Hence there is some orbit X when H acts on G/P of cardinality not divisible by p. 4

5 b). Let x X. By the stabilizer-orbit theorem, Stab H (x) X = H. Since p t divides H but not X, we must have p t Stab H (x) (since the integers form a UFD.) c). Let x X. If h Stab H (x), then hx = x; but now recall that x is a coset of G/P (if P is not normal, replace coset with left coset ), so we may also say, for any y in this coset, yhy 1 P. Since this holds for all h Stab H (x), we may deduce that yhy 1 P, i.e. H is conjugate (in G) to a subgroup of P. Any subgroup of P has order a power of p (by Lagrange s theorem) and so Stab H (x) has order a power of p. d). Let G be a finite group with a p-sylow subgroup P, and let H be a subgroup of G of order p t n as above; suppose further that p n. From the above, we have a subgroup S H (a stabilizer of a certain orbit X of the action of H on G/P ) whose order divisible by p t [from (b)] and whose order is a power of p [from (c)]. Since S divides H = p t n from Lagrange s theorem, we may conclude that S = p t, i.e. S is a p-sylow subgroup of H G. Since H may be any subgroup of G in the above, we are done. e). Any group G of order n acts on the vector space F n p as follows: choose a basis B of F n p and label them with the elements of the group, i.e. choose a bijection between B and G ; then let G act on the elements of B by left-multiplication (of the corresponding labels) and extend this action linearly to all of F n p. This produces a group map G GL n (F p ). This map is injective i.e. an embedding, as desired since the left-multiplication action described above is faithful (indeed, free, i.e. the action of any non-identity element has no fixed points.) f). (appears as second (e) on Homework) From the previous part (e), every finite group G embeds in GL n (F p ) for n = G and for every p. From #4(b), GL n (F p ) has a p-sylow subgroup. From part (d), G also has a p-sylow subgroup. g). (appears as (f) on Homework) Fix G be a finite group and let P 1 and P 2 be two p-sylow subgroups of G. If we run the above argument [(a) through (d)] with P = P 1 and H = P 2, then we see, by considerations of cardinality, that the stabilizer in question must be all of H = P 2, which is in turn conjugate to a subgroup of P 1 ; again, by considerations of cardinality, we may then conclude that P 2 is conjugate to P 1. 7 a). We may represent S n by the set of n n permutation matrices over k: if we let (e 1,..., e n ) denote a basis of k n, S n acts naturally on {e 1,..., e n } as a set of n objects (by permuta- 5

6 tions), and this extends uniquely (linearly) to an action of S n on k n, i.e. a group map S n GL n (k). We may verify that this group action is faithful, i.e. the group map S n GL n (k) is injective, and hence S n may be viewed as a subgroup of GL n (k) (by taking its isomorphic image in GL n (k) under this map.) If we fix (e 1,..., e n ) to be the standard basis for k n, the transposition (ij) corresponds to the elementary matrix obtained from the identity matrix by switch columns (or rows) i and j. Notice that each transposition has a determinant of -1. So define Det : S n {±1} and observe that this is a group map, and A n is its kernel. Thus, A n S n. b). We remark that alternating n-linear maps are skew n-linear. By skew-multilinearity and by the characterisation in (a) of the matrices representing transpositions, we note that det(τ ij ) = 1 for any transposition τ ij := (ij). Since det : S n GL n (k) k is a group map, the determinant of a product σ of n transpositions is given by the product of their determinants, i.e. det(σ) = ( 1) n. In particular, det(σ) = 1 if σ can be written as a product of an even number n of transpositions, and det(σ) = 1 if σ can be written as a product of an odd number of transpositions. If there is more than one way to write σ as a product of transpositions, then we must conclude that the numbers of transpositions in each of these products has the same parity, since the above argument must hold and det is a well-defined function. c). A n is normal. Take B = (12). Their intersection is trivial. All we need to show is S n = A n B. Enough to show that we can create any cycle (ab). For a 1, 2, b 1, 2 Use (ab)(12) (12). For (12) - done. For (1a) or (2a) use (12)(2a) (12) or (12)(1a) (12) respectively. 6

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