# Extra exercises for algebra

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1 Extra exercises for algebra These are extra exercises for the course algebra. They are meant for those students who tend to have already solved all the exercises at the beginning of the exercise session and who need an extra challenge. The difficulty of the exercises varies quite a bit. Most of them are not particularly hard, but introduce important concepts which the student might come across in later courses on algebra. Some of them aim to give an alternative proof of various results from the book or to just be a walk-through of a proof of some nice result not covered in the book. A few are, however, really tricky, and these have been marked as such. Exercise 1. The purpose of this exercise is to show that for any prime p, the group (Z/pZ) is cyclic. 1. Show that if G is an abelian group with element x and y such that gcd( x, y ) = 1 then xy = x y. 2. Show that if G is an abelian group, then for any two elements x and y in G, there is an element in G whose order is lcm( x, y ) (hint: Look at suitable powers of x and y to get elements whose orders are coprime). 3. Conclude that if m is the largest order among the elements of a finite abelian group G, and d is the order of some element of G, then d m. 4. Show that if p is a prime then (Z/pZ) is cyclic (hint: Let m be the largest order of an element in the group and consider the polynomial x m 1, then use the above together with Exercise 1.50 part (iii)). Exercise 2. Let G be a group and H G. Show that if ghg 1 H for all g G then H G. Note that the above is an easier condition to check than ghg 1 = H, which makes it more useful in practice. Exercise 3. Let D be the set Z { 1, 1} (ie, the set of all pairs (x, s) where x is an integer and s is either 1 or 1). Define a multiplication on D as follows: (x, s)(y, t) = (x + sy, st) (note that this makes sense since s will be either 1 or 1). Show that this makes D a group and show that any element of the form (x, 0) with x 0 has infinite order, while any element of the form (x, 1) has order 2. The group defined above is called the infinite dihedral group. The construction is a special case of what is known as a semidirect product, which is essentially the same as a direct product, but where the multiplication, instead of being (g, h)(g, h ) = (gg, hh ), is given in a way where the multiplication in the first coordinate is twisted in a way that depends on the second coordinate of the first factor (ie, h). To be more precise (skip this if it becomes too technical, it will not be needed for these exercises): Let G and H be groups and assume we have a homomorphism ϕ : H Aut(G) (for more about Aut(G), see exercise 5) Denote for any h H ϕ(h) = ϕ h. We define the semidirect product G ϕ H as follows: The underlying set is G H, and the multiplication is given by (g, h)(g, h ) = (gϕ h (g ), hh ). 1

2 The infinite dihedral group is now the case where G = Z, H = { 1, 1} (with usual multiplication), and the homomorphism ϕ : H Aut(G) sends 1 to the identity map and 1 to the automorphism that multiplies by 1. Exercise 4. Let G be a group and define the following subsets of G: S G = {g G g 2 = e}, T G = {g G g < } and U G = {ghg 1 h 1 g, h G} 1. Give an example of a group G where S G is not a subgroup of G. 2. Show that if S G is a subgroup of G, then S G is normal in G 3. Give an example of a group G where T G is not a subgroup of G (hint: Consider for example the infinite dihedral group (Exercise 3)). 4. Show that if T G is a subgroup of G, then T G is normal in G. 5. Show that if H is a subgroup of G and U G H, then H is normal in G and G/H is abelian. 6. Show that if H is a normal subgroup of G with G/H abelian, then U G H. The subsets considered above will appear again later, when we consider subgroups generated by subsets. Exercise 5. Let G be a group. An isomorphism ϕ : G G is called an automorphism of G. Let Aut(G) denote the set of automorphisms of G. 1. Show that Aut(G) with composition of functions is a group. 2. Show that G is abelian if and only if the map ϕ : G G with ϕ(g) = g 1 is an automorphism. 3. Show that for any n N \ {0} we have Aut(Z/nZ) (Z/nZ) (hint: For m Z/nZ with gcd(m, n) = 1 consider the map from Z/nZ to itself given by multiplication with m). 4. Show that Aut(Z) { 1, 1} (with the usual multiplication). 5. Let h G and define a map ϕ h : G G by ϕ h (g) = ghg 1. Show that ϕ h is an automorphism of G. Exercise 6. Let G be a group and H G. H is said to be characteristic in G if for all ϕ Aut(G) and all h H, we have ϕ(h) H. If H is characteristic in G, we write H ch G. 1. Show that if H ch G then H G (hint: Exercise 5 part 5). 2. Find an example of a group G with subgroups H and K such that K H and H G but K is not normal in G. 3. Show that if H, K are subgroups of G with K ch H and H ch G then K ch G. 4. Show that if H, K are subgroup of G with K ch H and H G then K G. 5. Find an example of a group G with subgroups H and K such that K H and H ch G but K is not normal in G. 2

3 6. Assume that H G and that for any subgroup K G with K H we have K H. Show that H ch G. 7. Assume that G is cyclic and H G. Show that H ch G. 8. Assume that P is a normal p-sylow subgroup of G for some prime p. Show that P ch G. 9. Show that Z(G) ch G. Exercise 7. Let G be a group and S G be a subset. Define S = H S H G So S is the intersection of all those subgroups of G that contain S. 1. Show that S = {g G n N (g 1,..., g n ) S n (i 1,..., i n ) Z n : g = g i 1 1 g i 2 2 gn in } (ie, the set of all possible products of powers (both positive and negative) of elements from S) (hint: Show that this set is a subgroup of G and that it is contained in any subgroup of G which contains S). 2. Show that if gsg 1 S for all g G and all s S, then S G. 3. Show that if ϕ(s) S for all ϕ Aut(G) and all s S then S ch G. 4. Show that the two above conditions can be weakened to gsg 1 S and ϕ(s) S and the conclusion still holds. 5. With notation as in Exercise 4, show that S G, T G and U G are characteristic subgroups of G. 6. Let V G = {g 2 g G} and let H = V G. Show that H ch G and that G/H is abelian. Exercise 8. Let G be a group. 1. Show that for any two elements x, y G, there is an n N and elements g 1,..., g n G such that xyx 1 y 1 = g 2 1 g2 2 g2 n (hint: Combine Exercise 7 part 6 with Exercise 4 part 6). 2. Given x, y G, find explicit elements g 1,..., g n G satisfying above equation (hint: These can be chosen to be combinations of x and y and their inverses, and you will not need too many of them). Exercise 9. Let G be a group and H a subgroup of G. We will in this exercise denote the centralizer of H in G by C G (H) = {g G gh = hg for all h H} (note that in the book, he denotes the centralizer of an element by Z(h), but the notation used in this exercise is more standard). For each g G we consider the automorphism ϕ g defined in part 5 of Exercise Consider the map ϕ : G Aut(G) given by ϕ(g) = ϕ g. Show that ϕ is a homomorphism and that ker(ϕ) = Z(G). 2. Show that for any h N G (H) we have that ϕ(h) Aut(H). Denote by ψ the restriction of ϕ to N G (H). 3

4 3. Show that ψ : N G (H) Aut(H) is a homomorphism and that ker(ψ) = C G (H). Conclude that C G (H) is a normal subgroup of N G (H) and that N G (H)/C G (H) is isomorphic to a subgroup of Aut(H). 4. Assume that p is the smallest prime dividing G and that p 2 does not divide G. Let P be a p-sylow subgroup of G. Show that N G (P ) = C G (P ) The reason for including the last part above is that there is a big result stating that if P is a p-sylow subgroup of G such that N G (P ) = C G (P ) then G has a normal subgroup of index P, and is hence not simple (unless G = p). Combining the above with the result that if only two distinct primes divide the order of G then G is not simple, one obtains a very direct way to see that the smallest possible order of a non-abelian simple group is 60. Exercise 10. Let G be a group and H G. H is said to be a maximal subgroup of G if H G and if there is no subgroup K of G with H K, K H and K G. 1. Show that if G is finite and K G with K G then there is a maximal subgroup H of G with K H. 2. Show that if G is a finite group with precisely one maximal subgroup, then G is cyclic. Exercise 11. Let G be a finite group and let H and K be subgroups of G. Show that HK = H K H K (note that this might not be a subgroup). Exercise 12. The purpose of this exercise is to give a solution to Exercise 2.52 (the result is known as Cauchy s theorem), without using the Sylow theorems. So we wish to show that if p is a prime which divides G, then G has a subgroup of order p. We will do this is several stages, by contradiction. So we assume from now on that G is a counterexample (so p is a prime that divides G but G has no subgroup of order p). We also assume that G has smallest possible order among counterexamples. 1. Show that if H G with H G then p does not divide H. 2. Show that G is not cyclic. 3. Assume that G is abelian and let H and K be distinct maximal subgroups of G (these exist by Exercise 10 and the above). Show that HK = G and that this is a contradiction. 4. Use the class equation to get a final contradiction (note that we can now assume G is not abelian). Exercise 13. Let G be a finite group. A subgroup H G is said to be a Hall-subgroup if gcd( H, G : H ) = Show that if P is a p-sylow subgroup of G for some prime p, then P is a Hall-subgroup of G. 2. Show that if H is a Hall-subgroup, then the order of H is uniquely determined by the set of primes that divide H. If π is the set of primes dividing H then H is said to be a π-hall subgroup. 3. Assume that H is a normal Hall-subgroup of G. Show that if K is a subgroup of G such that K divides H then K H (hint: Consider the product HK). 4

5 4. Conclude that if H is a normal π-hall subgroup of G, then it is the unique π-hall subgroup of G (compare to the similar result for p-sylow subgroups). Exercise 14. Let G be a finite group and let p be the smallest prime dividing G. Assume that H G with G : H = p. Show that H is normal in G (hint: Consider the action of G on the cosets of H and use this to get a homomorphism G S p ). Exercise 15 (Hard). Let G be a finite group and assume that for any subgroup H G with H G there is some prime p and some natural number n such that H = p n. The object of this exercise is to describe what such a G can look like when G is not the power of a prime. This will be done in a number of steps, some of them very challenging. The first step is not directly related to this problem, but it is a property of groups of prime power order, which will be needed in a later part. So assume that G is a group with the above property and such that at least two distinct primes divide G. Assume further that G is not cyclic. 1. Let H be a group with H = p n for some prime p and natural number n. Show that if K H with K H then N H (K) K (hint: Reduce to the case where K has index p in H and use Exercise 14). 2. Show that if G is simple and P and Q are p-sylow subgroups for some prime p with P Q, then P Q = {e} (hint: Let P and Q be distinct p-sylow subgroups whose intersection is as large as possible and consider N G (P Q)). 3. Conclude that G is not simple (hint: Use the above along with a counting argument via Sylow s theorems). 4. Show that if H G with H {1} and H G, then G/H = p for some prime p. 5. Show that there is a unique normal subgroup H of G with H {1} and H G. From now on, this subgroup will continue to be called H. 6. Conclude that G = pq n for two distinct primes p and q and some n Show that H is abelian. 8. Show for all h H we have h q = Show that p divides q n Show that p divides qn 1 q Show that p n. (somewhat harder than the previous one). 5

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