# School of Mathematics and Statistics. MT5824 Topics in Groups. Problem Sheet I: Revision and Re-Activation

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1 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet I: Revision and Re-Activation 1. Let H and K be subgroups of a group G. Define HK = {hk h H, k K }. (a) Show that HK is a subgroup of G if and only if HK = KH. (b) Show that if K is a normal subgroup of G, then HK is a subgroup of G. (c) Give an example of a group G and two subgroups H and K such that HK is not a subgroup of G. (d) Give an example of a group G and two subgroups H and K such that HK is a subgroup of G but neither H nor K are normal subgroups of G. 2. Let M and N be normal subgroups of G. Show that M N and MN are normal subgroups of G. 3. Let G be a group and H be a subgroup of G. (a) If x and y are elements of G, show that Hx = Hy if and only if x Hy. (b) Suppose T is a subset of G containing precisely one element from each (right) coset of H in G (such a set T is called a (right) transversal to H in G and has the property that T = G : H ). Deduce that {Ht t T } is the set of all (right) cosets of H in G with distinct elements of T defining distinct cosets. 1

2 4. Let G be a (not necessarily finite) group with two subgroups H and K such that K H G. The purpose of this question is to establish the index formula G : K = G : H H : K. Let T be a transversal to K in H and U be a transversal to H in G. (a) By considering the coset Hg or otherwise, show that if g is an element of G, then Kg = Ktu for some t T and some u U. (b) If t,t T and u,u U with Ktu = Kt u, first show that Hu = Hu and deduce u = u, and then show that t = t. (c) Deduce that TU = {tu t T, u U } is a transversal to K in G and that G : K = G : H H : K. (d) Show that this formula follows immediately from Lagrange s Theorem if G is a finite group. 5. Let G be a group and H be a subgroup of G. (a) Show that H is a normal subgroup of G if and only if Hx = xh for all x G. (b) Show that if G : H = 2, then H is a normal subgroup of G. 6. Give an example of a finite group G and a divisor m of G such that G has no subgroup of order m. 7. Let G = x be a cyclic group. (a) If H is a non-identity subgroup of G, show that H contains an element of the form x k with k > 0. Choose k to be the smallest positive integer such that x k H. Show that every element in H has the form x kq for some q Z and hence that H = x k. [Hint: Use the Division Algorithm.] Deduce that every subgroup of a cyclic group is also cyclic. (b) Suppose now that G is cyclic of order n. Let H be the subgroup considered in part (a), so that H = x k where k is the smallest positive integer such that x k H, and suppose that H = m. Show that k divides n. [Hint: Why does x n H?] Show that o(x k ) = n/k and deduce that m = n/k. Conclude that, if G is a cyclic group of finite order n, then G has a unique subgroup of order m for each positive divisor m of n. (c) Suppose now that G is cyclic of infinite order. Let H be the subgroup considered in part (a), so that H = x k where k is the smallest positive integer such that x k H. Show that {1,x,x 2,...,x k 1 } is a transversal to H in G. Deduce that G : H = k. [Hint: Use the Division Algorithm to show that if n Z, then x n Hx r where 0 r < k.] Conclude that, if G is a cyclic group of infinite order, then G has a unique subgroup of index k for each positive integer k and that every non-trivial subgroup of G is equal to one of these subgroups. 2

3 8. Let V 4 denote the Klein 4-group: that is V 4 = {1,a,b,c} where a = (12)(34), b = (13)(24) and c = (14)(23) (permutations of four points). Find three distinct subgroups H 1, H 2 and H 3 of V 4 of order 2. Show that H i H j = 1 for all i j and V 4 = H i H j for all i and j. [Note that I am using the more conventional notation V 4 for the Klein 4-group, rather than the less frequently used K 4 from MT4003. Here V stands for Viergruppe.] 9. The dihedral group D 2n of order 2n is generated by the two permutations α = ( n), β = (2n)(3n 1). (a) Show that α generates a normal subgroup of D 2n of index 2. (b) Show that every element of D 2n can be written in the form α i β j where i {0,1,...,n 1} and j {0,1}. (c) Show that every element in D 2n which does not lie in α has order The quaternion group Q 8 of order 8 consists of eight elements with multiplication given by 1, 1,i, i,j, j,k, k i 2 = j 2 = k 2 = 1, ij = ji = k, jk = kj = i, ki = ik = j. (a) Show that Q 8 is generated by i and j. (b) Show that i is a normal subgroup of Q 8 of index 2. (c) Show that every element of Q 8 can be written as i m j n where m {0,1,2,3} and n {0,1}. (d) Show that every element in Q 8 which does not lie in i has order 4. (e) Show that Q 8 has a unique element of order 2. 3

4 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet II: Group Actions 1. (a) How many different ways can the cyclic group C 3 of order three act on the set {1,2,3,4}? (b) How many different ways can the cyclic group C 4 of order four act on the set {1, 2, 3}? [Consider orbit decompositions and apply the Orbit-Stabiliser Theorem.] 2. (a) Let i 1, i 2,..., i k be distinct points in Ω = {1,2,...,n} and let σ be a permutation in S n. By considering the effect on various points in Ω, or otherwise, show that σ 1 (i 1 i 2... i k )σ = (i 1 σ i 2 σ... i k σ). Deduce that two permutations in S n are conjugate if and only if they have the same cycle structure. (b) Give a list of representatives for the conjugacy classes in S 5. How many elements are there in each conjugacy class? Hence calculate the order of and generators for the centralisers of these representatives. 3. Let G be a group and let Γ and be sets such that G acts on Γ and on. Define (γ,δ) x = (γ x,δ x ) for all γ Γ, δ and x G. Verify that this is an action of G on the set Γ. Verify that the stabiliser of the pair (γ,δ) in this action equals the intersection of the stabilisers G γ and G δ (these being the stabilisers under the actions of G on Γ and, respectively). If G acts transitively on the non-empty set Ω, show that {(ω,ω) ω Ω } is an orbit of G on Ω Ω. Deduce that G acts transitively on Ω Ω if and only if Ω = (a) There is a natural action of S n on Ω = {1,2,...,n}. Show this action is transitive. How many orbits does S n have on Ω Ω? (b) Repeat part (a) with the action of the alternating group A n on Ω. 1

5 5. Let G be a group and H be a subgroup of G. Show that the normaliser N G (H) of H is the largest subgroup of G in which H is a normal subgroup. [By largest, we mean that if L is any subgroup of G in which H is normal, then L N G (H). So you should check that (i) H N G (H) and (ii) if H L then L N G (H).] 6. Let G be a group and H be a subgroup of G. Let Ω be the set of right cosets of H in G. Define an action of G on Ω by for Hg Ω and x G. Ω G Ω (Hg,x) Hgx (a) Verify that this action is well-defined and that it is indeed a group action. (b) Is the action transitive? (c) Show that the stabiliser of the coset Hx is the conjugate H x of H. (d) Let ρ: G Sym(Ω) be the permutation representation associated to the action of G on Ω. Show that ker ρ = H x. (e) Show that ker ρ is the largest normal subgroup of G contained in H. [That is, show that (i) it is a normal subgroup of G contained in H and (ii) if K is any normal subgroup of G contained in H then K ker ρ. This kernel is called the core of H in G and is denoted by Core G (H).] 7. If H is a subgroup of G of index n, show that the index of the core of H in G divides n!. 8. Let G be a group and let G act on itself by conjugation. Show that the kernel of the associated permutation representation ρ: G Sym(G) is the centre Z(G) of G. Deduce that Z(G) is a normal subgroup of G. 9. Let G be a group and let Aut G denote the set of all automorphisms of G. Show that AutG forms a group under composition. For g G, let τ g : G G be the map given by conjugation by g; that is, x G τ g : x g 1 xg for all x G. Show that the map τ : g τ g is a homomorphism τ : G AutG. What is the kernel of τ? Write InnG for the image of τ. Thus InnG is the set of inner automorphisms of G. Show that InnG is a normal subgroup of AutG. [Hint: Calculate the effect of φ 1 τ g φ on an element x, where φ AutG, τ g InnG and x G.] 2

6 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet III: Cauchy s Theorem and Sylow s Theorem 1. Let H be a subgroup of the symmetric group S n of index 2. Show that H = A n. [Hint: Show that H contains all squares of elements in S n.] 2. Let G be a finite group, let p be a prime number and write G = p n m where p does not divide m. The purpose of this question is to use group actions to show G has a subgroup of order p n ; that is, G has a Sylow p-subgroup. (a) Let Ω be the collection of all subsets of G of size p n : Show that ( p n ) m Ω = = m p n ( p n m 1 p n 1 Ω = {S G S = p n }. )( p n ) m 2 p n... 2 ( p n m p n )( + 2 p n m p n + 1 (b) Let j be an integer with 1 j p n 1. Show that if the prime power p i divides j, then p i divides p n j. Conversely show that if p i divides p n j, then p i divides j. Deduce that Ω is not divisible by p. [Examine each factor in the above formula and show that the power of p dividing the numerator coincides with the power dividing the denominator.] (c) Show that we may define a group action of G on Ω by Ω G Ω (S,x) Sx = {ax a S }. (d) Express Ω as a disjoint union of orbits: Ω = Ω 1 Ω 2 Ω k. Show that p does not divide Ω i for some i. (e) Let S Ω i and let P = G S, the stabiliser of S in the action of G on Ω. Show that p does not divide G : P and deduce that p n divides P. (f) Fix a 0 S. Explain why a 0 x S for all x P. Show that x a 0 x is an injective map P S. Deduce that P p n. (g) Conclude that P is a Sylow p-subgroup of G. 2 1 ). 1

7 3. Show that there is no simple group of order equal to each of the following numbers: (i) 30; (ii) 48; (iii) 54; (iv) 66; (v) 72; (vi) 84; (vii) 104; (viii) 132; (ix) 150; (x) 392. [Note: These are not necessarily in increasing order of difficulty!] 4. Let G be a finite group, N be a normal subgroup of G and P be a Sylow p-subgroup of G. (a) Show that P N is a Sylow p-subgroup of N. (b) Show that PN/N is a Sylow p-subgroup of G/N. [Hint: Show that the subgroup is of order a power of p and has index not divisible by p. In both parts expect to use the formula for the order of PN and the fact that P already has the required property as a subgroup of G.] 5. Let G be a finite group, p be a prime number dividing the order of G, and let P be a Sylow p-subgroup of G. Define O p (G) = P g. g G Show that O p (G) is the largest normal p-subgroup of G. 2

8 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet IV: Composition series and the Jordan Hölder Theorem 1. Let G be a group and N be a normal subgroup of G. If G/N and N both have composition series, show that G has a composition series and that the set of composition factors of G equals the union of those of G/N and those of N. [Use the Correspondence Theorem to lift the terms of a composition series to a chain of subgroups between G and N.] 2. Let p, q and r be prime numbers with p < q < r. Let G be a group of order pqr. (a) Suppose that G does not have a unique Sylow r-subgroup. Show that it has a unique Sylow q-subgroup Q. [Hint: How many elements of order r must there be and how many elements of order q if there were no normal Sylow q-subgroup?] (b) Show that G/Q has a unique Sylow r-subgroup of the form K/Q where K G and K = qr. (c) Show that K has a unique Sylow r-subgroup and deduce that, in fact, G has a unique Sylow r-subgroup R (contrary to the assumption in (a)). [Hint: Why is a Sylow r-subgroup of K also a Sylow r-subgroup of G? Remember that all the Sylow r-subgroups of G are conjugate.] (d) Show that G/R has a unique Sylow q-subgroup. (e) Deduce that G has a composition series G = G 0 > G 1 > G 2 > G 3 = 1 where G 1 = qr and G 2 = r. Up to isomorphism, what are the composition factors of G? 3. Let p be a prime number and let G be a non-trivial p-group. Show that G has a chain of subgroups G = G 0 > G 1 > G 2 > > G n = 1 such that G i is a normal subgroup of G and G : G i = p i for i = 0, 1,..., n. What are the composition factors of G? [Hint: Use the fact that Z(G) 1 to produce an element x Z(G) of order p. Consider the quotient group G/K where K = x and apply induction.] 1

9 4. The dihedral group D 8 has seven different composition series. Find all seven. 5. How many different composition series does the quaternion group Q 8 have? 6. Let G be a group, N be a normal subgroup of G and suppose that G = G 0 > G 1 > G 2 > > G n = 1 is a composition series for G. Define N i = N G i for i = 0, 1,..., n. (a) Show that N i+1 is a normal subgroup of N i for i = 0, 1,..., n 1. (b) Use the Second Isomorphism Theorem to show that N i /N i+1 = (G i N)G i+1 G i+1. [Hint: Note that N i+1 = G i+1 (G i N).] (c) Show that (G i N)G i+1 is a normal subgroup of G i containing G i+1. Deduce that (G i N)G i+1 /G i+1 is either equal to G i /G i+1 or to the trivial group. [Remember G i /G i+1 is simple.] (d) Deduce that N possesses a composition series. [Hint: Delete repeats in the series (N i ).] 7. Let G be a group, N be a normal subgroup of G and suppose that G = G 0 > G 1 > G 2 > > G n = 1 is a composition series for G. Define Q i = G i N/N for i = 0, 1,..., n. (a) Show that Q i is a subgroup of G/N such that Q i+1 is a normal subgroup of Q i for i = 0, 1,..., n 1. (b) Use Dedekind s Modular Law to show that G i+1 N G i = (G i N)G i+1. Show that Q i /Q i+1 = G i /G i+1 (G i N)G i+1 /G i+1. [Hint: Use the Third Isomorphism Theorem, the Second Isomorphism Theorem and note that G i N = G i (G i+1 N) since G i+1 G i.] (c) Show that (G i N)G i+1 is a normal subgroup of G i containing G i+1. Deduce that (G i N)G i+1 /G i+1 is either equal to G i /G i+1 or to the trivial group. [Remember G i /G i+1 is simple.] Hence show that the quotient on the right hand side in (b) is either trivial or isomorphic to G i /G i+1. (d) Deduce that G/N possesses a composition series. 2

10 8. The purpose of this question is to prove the Jordan Hölder Theorem. Let G be a group and suppose that and Proceed by induction on n. G = G 0 > G 1 > > G n = 1 G = H 0 > H 1 > > H m = 1. (a) If n = 0, observe that G = 1 and that the Jordan Hölder Theorem holds in this (vacuous) case. (b) Suppose n 1. Consider the case when G 1 = H 1. Observe the conclusion of the theorem holds for this case. (c) Now suppose G 1 H 1. Use the fact that G 0 /G 1 and H 0 /H 1 are simple to show that G 1 H 1 = G. [Hint: Observe that it is a normal subgroup containing both G 1 and H 1.] (d) Define D = G 1 H 1. Show that D is a normal subgroup of G such that G 0 /G 1 = H1 /D and H 0 /H 1 = G1 /D. (e) Use Question 6 to see that D possesses a composition series D = D 2 > D 3 > > D r = 1. (f) Observe that we now have four composition series for G: G = G 0 > G 1 > G 2 > > G n = 1 G = G 0 > G 1 > D = D 2 > > D r = 1 G = H 0 > H 1 > D = D 2 > > D r = 1 G = H 0 > H 1 > H 2 > > H m = 1. Apply the case of part (b) (twice) and the isomorphisms of part (d) to complete the induction step of the proof. 9. Let G be a simple group of order 60. (a) Show that G has no proper subgroup of index less than 5. Show that if G has a subgroup of index 5, then G = A 5. [Hint for both parts: If H is a subgroup of index k, act on the set of cosets to produce a permutation representation. What do we know about the kernel?] (b) Let S and T be distinct Sylow 2-subgroups of G. Show that if x S T then C G (x) 12. Deduce that S T = 1. [Hint: Why are S and T abelian?] (c) Deduce that G has at most five Sylow 2-subgroups and hence that indeed G = A 5. [Hint: The number of Sylow 2-subgroups equals the index of a normaliser.] Thus we have shown that there is a unique simple group of order 60 up to isomorphism. 3

11 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet V: Direct products and semidirect products 1. Give an example of two groups G and H and a subgroup of the direct product G H which does not have the form G 1 H 1 where G 1 G and H 1 H. 2. Let M and N be normal subgroups of a group G. By considering the map g (Mg,Ng), or otherwise, show that G/(M N) is isomorphic to a subgroup of the direct product G/M G/N. 3. Using Question 2, or otherwise, show that if m and n are coprime integers, then C m C n = Cmn. 4. Let X 1, X 2,..., X n be non-abelian simple groups and let G = X 1 X 2 X n. (In this question we shall identify the concepts of internal and external direct products. Thus we shall speak a subgroup of G containing a direct factor X i when, if we were to use the external direct product notation strictly, we might refer to it containing the subgroup X i in the notation of the lectures.) Prove that a non-trivial normal subgroup of G necessarily contains one of the direct factors X i. Hence show that every normal subgroup of G has the form X i1 X i2 X ik for some subset {i 1,i 2,...,i k } of {1,2,...,n}. [Hint: If N is a non-trivial normal subgroup of G, choose a non-identity element (x 1,x 2,...,x n ) N. Consider conjugating this element by the element (1,...,1,g,1,...,1).] Now suppose X 1, X 2,..., X n are abelian simple groups. Is it still true that every normal subgroup of the direct product has this form? 1

12 5. Let p be a prime number. (a) Show that Aut C p = Cp 1. (b) Show that Aut(C p C p ) = GL 2 (F p ) (where F p = Z/pZ denotes the field of p elements). [For (a), let C p = x. Observe that an automorphism α is given by x x m where m is a representative for a non-zero element of F p. Recall the multiplication group of a finite field is cyclic. For (b), write C p C p additively and view it as a vector space over F p. Show that automorphisms of the group then correspond to invertible linear maps.] 6. Let G = x be a cyclic group. Show that AutG is abelian. 7. Show that the dihedral group D 2n is isomorphic to a semidirect product of a cyclic group of order n by a cyclic group of order 2. What is the associated homomorphism φ: C 2 Aut C n? 8. Show that the quaternion group Q 8 may not be decomposed (in a non-trivial way) as a semidirect product. [Hint: How many elements of order 2 does Q 8 contain?] 9. Show that the symmetric group S 4 of degree 4 is isomorphic to a semidirect product of the Klein 4-group V 4 by the symmetric group S 3 of degree 3. Show that S 4 is also isomorphic to a semidirect product of the alternating group A 4 by a cyclic group of order Let G be a group of order pq where p and q are primes with p < q. (a) If p does not divide q 1, show that G = C pq, the cyclic group of order pq. (b) If p does divide q 1, show that there are essentially two different groups of order pq. 11. Classify the groups of order 52 up to isomorphism. 12. Show that a group of order 30 is isomorphic to one of C 30, C 3 D 10, D 6 C 5, D Classify the groups of order 98 up to isomorphism. 14. Classify the groups of order 117 up to isomorphism. 2

13 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet VI: Soluble groups 1. Let G and H be groups. (a) Show that (G H) = G H. Deduce that (G H) (i) = G (i) H (i) for all i 0. (b) Deduce that the direct product of two soluble groups is a soluble group. Could this result be deduced from other results proved in lectures? 2. Calculate the terms in the derived series for the following groups: (i) S 3 ; (ii) D 8 ; (iii) A 4 ; (iv) S 4 ; (v) A 5 ; (vi) S 5. [It may well help to remember that G is the smallest normal subgroup of G such that G/G is abelian.] 3. Let p be a prime number and let G be a finite p-group. Show that G is soluble. [Hint: Proceed by induction, using the fact that Z(G) 1 for a non-trivial p-group G.] 4. Show that 1 and G are always characteristic subgroups of a group G. Give an example of a normal subgroup N of a group G such that N is not a characteristic subgroup of G. 5. Let G be a finite group and suppose that P is a Sylow p-subgroup of G (for some prime p) which is actually normal in G. Prove that P is a characteristic subgroup of G. [Hint: Where does an automorphism map a Sylow p-subgroup?] 6. Let G be any group and let n be an integer. Define G n = g n g G, the subgroup of G generated by all its nth powers. Show that G n is a characteristic subgroup of G. Show that every element in the quotient group G/G n has order dividing n. 1

14 7. (a) Give an example of a group G with subgroups K and H such that K H, H G but K G. (b) Give an example of a group G with a characteristic subgroup H and a homomorphism φ: G K such that Hφ is not a characteristic subgroup of Gφ. (c) Give an example of a group G with subgroups H and L such that H L G, H char G, but H is not a characteristic subgroup of L. (d) Give an example of a group G with subgroups K and H such that K H, H is a characteristic subgroup of G but K G. [Hint: A little thought should tell you that G 8 is required for many of these examples. There are examples without making the groups considerably larger than this minimum.] 8. (a) Let M and N be normal subgroups of a group G that are both soluble. Show that MN is soluble. [Hint: M MN. What can you say about the quotient MN/M?] (b) Deduce that a finite group G has a largest normal subgroup S which is soluble. [ Largest in the usual sense of containing all others. This normal subgroup S is called the soluble radical of G.] (c) Prove that S is the unique normal subgroup of G such that S is soluble and G/S has no non-trivial abelian normal subgroup. [Hint: To show G/S has no non-trivial abelian normal subgroup, remember that if H is a group with N H such that H/N and N are soluble, then H is soluble. Use N = S and choose H such that H/S is abelian.] 9. (a) Let M and N be normal subgroups of a group G such that G/M and G/N are both soluble. Show that G/(M N) is soluble. [Hint: Use Question 2 on Problem Sheet V.] (b) Deduce that a finite group G has a smallest normal subgroup R such that the quotient G/R is soluble. [ Smallest in the usual sense of being contained in all others. This normal subgroup R is called the soluble residual of G.] (c) Prove that R is the unique normal subgroup of G such that G/R is soluble and R = R. [Hint: To show R = R, remember that if H is a group with N H such that H/N and N are soluble, then H is soluble. Use H = G/R and N = R/R.] 2

15 10. The following is an alternative way of proving that a minimal normal subgroup of a finite soluble group is an elementary abelian p-group. Let G be a finite soluble group and M be a minimal normal subgroup of G. (a) By considering M, prove that M is abelian. [Hint: M char M.] (b) By considering a Sylow p-subgroup of M, prove that M is a p-group for some prime p. (c) By considering the subgroup of M generated by all elements of order p, prove that M is an elementary abelian p-group. 11. Let G be the semidirect product of a cyclic group N of order 35 by a cyclic group C of order 4 where the generator of C acts by inverting the generator of N: G = N C = x,y x 35 = y 4 = 1, y 1 xy = x 1. Find a Hall π-subgroup H of G, its normaliser N G (H) and state how many Hall π-subgroups G possesses when (i) π = {2,5}, (ii) π = {2,7}, (iii) π = {3,5} and (iv) π = {5,7}. 12. Let p, q and r be distinct primes with p < q < r. Let G be a group of order pqr. (a) Show that G is soluble. (b) Show that G has a unique Hall {q,r}-subgroup. (c) How many Hall {p,r}-subgroups can G have? Can you construct examples to show that these numbers are indeed possible? [Hints: Review Question 2 on Problem Sheet IV. Consider the normaliser of the Hall subgroup. Semidirect products are good ways to construct groups with normal subgroups.] 13. A maximal subgroup of G is a proper subgroup M of G such that there is no subgroup H strictly contained between M and G. If p divides the order of the finite soluble group G, show that there is a maximal subgroup of G whose index is a power of p. [Hint: Consider Hall p -subgroups.] Show, by an example, that this is false for insoluble groups. 14. Let G be a finite soluble group whose order is divisible by k distinct prime numbers. Prove there is a prime p and a Hall p -subgroup H such that G H k/(k 1). [Hint: Consider G = p n 1 1 pn pn k k and consider the smallest value of p n i i.] 3

16 15. The purpose of this question is to prove Theorem 6.33; that is, that any two Sylow bases in a finite soluble group G are conjugate. Let G be a finite soluble group and let p 1, p 2,..., p k be the distinct prime factors of G. Let S i be the set of Hall p i-subgroups of G (for i = 1, 2,..., k) and let S be the collection of all Sylow bases of G; that is, the elements of S are sequences (P i ) such that P i is a Sylow p i -subgroup of G for i = 1, 2,..., k and P i P j = P j P i for all i and j. (a) Show that ( (Q 1,Q 2,...,Q k ) j, j 1Q j 2Q j,..., ) Q j j k is a bijection from S 1 S 2 S k to S. (b) Now fix a representative Q i in S i. Show that S i = G : N G (Q i ). Deduce that S i is a power of the prime p i. (c) Show that G acts on S according to the rule: ( (Pi ),g ) (P g i ) for (P i ) S and g G. (As usual, P g i denotes the conjugate of P i by g.) (d) Now concentrate on the specific Sylow basis (P i ) constructed from the Q i as in lectures. [Also compare part (a).] Show that the stabiliser of (P i ) under the above action is the intersection k i=1 N G(P i ) of the normalisers of the P i, and that this coincides with the intersection k j=1 N G(Q j ). (e) Use part (b) to show that [Hint: Coprime indices!] k G : k N G (Q j ) = G : N G (Q j ). j=1 j=1 (f) Use part (a) to deduce that G acts transitively on S. 4

17 MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet VII: Nilpotent groups 1. Show that γ 2 (G) = G. Deduce that abelian groups are nilpotent. 2. Show that Z(S 3 ) = 1. Hence calculate the upper central series of S 3 and deduce that S 3 is not nilpotent. Show that γ i (S 3 ) = A 3 for all i 2. [Hint: We have calculated S 3 previously and now know that S 3 is not nilpotent.] Find a normal subgroup N of S 3 such that S 3 /N and N are both nilpotent. 3. Show that Z(G H) = Z(G) Z(H). Show, by induction on i, that Z i (G H) = Z i (G) Z i (H) for all i. Deduce that a direct product of a finite number of nilpotent groups is nilpotent. 4. Let G be an finite elementary abelian p-group. Show that Φ(G) = Let G be a finite p-group. If M is a maximal subgroup of G, show that G : M = p. [Hint: G is nilpotent, so M G.] Deduce that G p G Φ(G). Use the previous question to show that Φ(G) = G p G. Show that G can be generated by precisely d elements if and only if G/Φ(G) is a direct product of d copies of the cyclic group of order p. 6. Let G be a nilpotent group with lower central series G = γ 1 (G) > γ 2 (G) > > γ c (G) > γ c+1 (G) = 1. Suppose N is a non-trivial normal subgroup of G. Choose i to be the largest positive integer such that N γ i (G) 1. Show that [N γ i (G),G] = 1. Deduce that N Z(G) 1. 1

### School of Mathematics and Statistics. MT5824 Topics in Groups. Handout 0: Course Information

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