# Homework #11 Solutions

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1 Homework #11 Solutions p 166, #18 We start by counting the elements in D m and D n, respectively, of order 2. If x D m and x 2 then either x is a flip or x is a rotation of order 2. The subgroup of rotations in D m is cyclic of order m, and since m is even there is exactly φ2 1 rotation of order 2. Therefore, D m contains exactly m + 1 elements of order 2. On the other hand, y D n can have order 2 only if y is a flip, since the rotations in D n have order dividing n, which is odd. Therefore there are exactly n elements in D n of order 2. We are now in a position to count the elements of order 2 in D m D n. Suppose x, y D m D n and x, y 2. Since x, y lcm x, y, it must be that either x 2 and y 1, 2 or x 1 and y 2. In the first case, the preceding paragraph shows that there are m + 1 choices for x and n + 1 choices for y, giving a total of m + 1n + 1 pairs. In the second case x e and there are n choices for y, yielding another n pairs. Thus, the total number of pairs with order 2 is m + 1n n. p 166, #28 It is not hard to show that Z 12 Z 4 Z 15 has no element of order 9, so we won t be able to find a cyclic subgroup of order 9. We therefore look for the next easiest type of subgroup, namely one of the form H K J where H Z 12, K Z 4 and J Z 15. The order of such a subgroup is H K J. If this is to equal 9, Lagrange s theorem tells us that we need H 3, K 1 and J 3. Since Z 12, Z 4 and Z 15 are all cyclic, they have unique subgroups of these orders. That is, we must take H 4, K {0} and J 5, so our subgroup is 4 {0} 5 p 167, #40 According to Corollary 1 of Theorem 8.2 we have Z 10 Z 12 Z 6 Z2 Z 5 Z 12 Z 6 Z2 Z 60 Z 6 Z60 Z 6 Z 2. p 167, #44 We have mod 15 8 φ2, 3 φ8 2 mod 3, 8 3 mod 5 φ1, 4

2 which shows that 1, 4 maps to 1. p 167, #50 Since , the Corollary to Theorem 8.3 gives U165 U3 U5 U11 Z 2 Z 4 Z 10. p 167, #52 We begin by observing that AutZ 20 U20 U4 U5 Z 2 Z 4. If x, y Z 2 Z 4 has order 4 then, since x, y lcm x, y, x is free and y must have order 4. Since Z 4 has φ4 2 elements of order 4, it follows that Z 2 Z 4, and hence AutZ 20, has 4 elements of order 4. On the other hand, since 4 x, y 0, 0 for every x, y Z 2 Z 4, Lagrange s theorem tells us that the possible orders of elements are 1, 2 or 4. Having counted the order 4 elements, and knowing that only the identity has order 1, we conclude that there must be exactly 3 elements of order 2. p 168, #58 By the Corollary to Theorem 8.3: U144 U U2 4 U3 2 Z 2 Z 4 Z 6 U140 U U2 2 U5 U7 Z 2 Z 4 Z 6 proving that U144 U140. p 191, #4 H is not normal in GL2, R since A H, B 0 1 and BAB 1 which does not belong to H GL2, R p 191, #10 Let G a be a cyclic group and let H G. If gh G/H then g a n for some n Z so that gh a n H ah n ah. This shows that G/H ah and is hence cyclic. p 191, #12 Let G be an abelian group and let H G. For any ah, bh G/H we have, since G is abelian, ahbh abh bah bhah

3 which proves that G/H is abelian as well. p 191, #14 Since 8 {0, 8, 16} and 2 14 mod mod mod 24 8 we see that the coset has order 4 in Z 24 / 8. p 192, #18 Since 15 has order 4 in Z 60, Lagrange s theorem tells us that Z 60 / 15 [Z 60 : 15 ] Z p 192, #22 We start by noting that 2, 2 {2m, 2m m Z} so that n 1, 0 n, 0 2, 2 for every n Z +. From this it follows that 1, 0 + 2, 2 must have infinite order in Z Z/ 2, 2 and hence that this group has infinite order. If Z Z/ 2, 2 were cyclic, it would have to be isomorphic to Z, the only infinite cyclic group. However, Z has no elements of order 2, whereas 1, 1 + 2, 2 is an element of Z Z/ 2, 2 with order 2. Consequently, Z Z/ 2, 2 is not isomorphic to Z and so is not cyclic. p 192, #26 Using the Cayley table for G on page 90 we find that the 4 cosets of H are H {e, a 2 } ah {a, a 3 } bh {b, ba 2 } bah {ba, ba 3 }. Moreover, according to the same Cayley table we have ah 2 a 2 H H bh 2 b 2 H a 2 H H so that G/H has at least 2 distinct elements of order 2. Since Z 4 has only a single element of order 2, it must be that G/H Z 2 Z 2. p 192, #28 The four cosets in G/H are H {0, 0, 2, 0, 0, 2, 2, 2} 1, 1 + H {1, 1, 3, 1, 1, 3, 3, 3} 1, 2 + H {1, 2, 3, 2, 1, 0, 3, 0} 2, 1 + H {2, 1, 0, 1, 2, 3, 0, 3}

4 which all have order 2. Therefore G/H Z 2 Z 2. The cosets in G/K are K {0, 0, 1, 2, 2, 0, 3, 2} 1, 1 + K {1, 1, 2, 3, 3, 1, 0, 3} 1, 3 + K {1, 3, 2, 1, 3, 3, 0, 1} 2, 2 + K {2, 2, 3, 0, 0, 2, 1, 0} and since 1, 1 + K clearly has order 4, it must be that G/K Z 4. p 193, #42 We prove only the generalization, which is the following. Proposition 1. Let G be a group and let n Z +. If G has a unique subgroup of order n then that subgroup is normal in G. Proof. Let H be the unique subgroup of G of order n. For any x G, xhx 1 is also a subgroup of G with order n. Therefore it must be that xhx 1 H. Since x G was arbitrary, this proves that H is normal in G. p 193, #44 We prove only the generalization, which is the following. Proposition 2. If G is a finite group then [G : ZG] is 1 or is composite. Proof. Assume, for the sake of contradiction, that [G : ZG] is prime. Then G/ZG is a group of prime order and is therefore cyclic. Theorem 9.3 then implies that G is abelian, which means that G ZG and so [G : ZG] 1, which is a contradiction. p 193, #46 Let ah G/H. If ah has finite order then there is an n Z + so that a n H ah n H, i.e. a n H. But every element of H has finite order and so there is an m Z + so that a nm a n m e which implies, since mn 1, that a has finite order. That is, a H so that ah is the trivial coset H. We have therefore shown that the only element of G/H with finite order is the identity, which is equivalent to the desired conclusion. p 195, #70 We begin with the following general result. Proposition 3. Let G be a group and H G. For any g G the set K i Z g i H is a subgroup of G.

5 Proof 1. We use the one-step subgroup test. We begin by noting that K since H K and H. If x, y K then there exist h 1, h 2 H and i, j Z so that x g i h 1 and y g j h 2. Since H G, g j H Hg j, and so h 1 h 1 2 g j g j h 3 for some h 3 H. Thus xy 1 g i h 1 h 1 2 g j g i g j h 3 g i j h 3 K proving that K passes the one-step subgroup test. Proof 2. Let γ : G G/H be the natural homomorphism. Since the kernel of γ is H and γg i g i H gh i, γ 1 gh i g i H by Theorem Thus K i Z g i H i Z γ 1 gh i γ 1 i Z{gH i } γ 1 gh which shows that K is a subgroup of G by Theorem The conclusion of the problem now follows easily. Since gh has order 3, the cosets H, gh and g 2 H are distinct, and any other coset of the form g i H is one of these. Therefore g i H H gh g 2 H i Z and the latter set contains exactly 12 elements since H 4. The proposition tells us this set is a subgroup of G, so we re finished. p 210, #6 Let f, g G. The linearity of differentiation assures us that f + g is an antiderivative of f + g, i.e. f + g f + g f + g. Furthermore, since f0 g0 0 we have f + g 0 f 0 + g so that f + g passes through the point 0, 0. It follows from the definition of that f + g f + g, proving that the map f f is indeed a homomorphism. If we require that the antiderivative f pass through any point a, b with b 0 then the map is never a homomorphism. To see this, note that for any f G we have f a b and f + f a f a + f a 2b b f + f a demonstrating that f + f f + f.

6 p 211, #10 Let x, y G. To show that φxy φxφy we consider 4 possible cases. Case 1: x and y are both rotations. Then xy is also a rotation and so φxφy φxy. Case 2: x is a rotation and y is a reflection. Then xy is also a reflection and so φxφy φxy. Case 3: x is a reflection and y is a rotation. Then, as above, xy is a reflection and so φxφy φxy. Case 4: x and y are both reflections. Then xy is a rotation and so φxφy φxy. Since φxy φxφy in each case, we conclude that φ is a homomorphism. It s clear from the definition of φ that ker φ consists of all of the rotations in G, i.e. ker φ G R 360/n, where G D n. Note that this proves that for any subgroup G of a dihedral group, the set of rotations in G is a normal subgroup of G. p 211, #14 This function is not a homomorphism because it fails to preserve the respective group operations. To be specific, if we denote the function by φ, we have and That is, φ6 + 6 φ6 + φ6. φ6 + 6 φ0 0 φ6 + φ mod p 212, #24a Since φ7 6 and 43 6 mod 50 1 we have φ1 φ φ mod 15 3 from which it follows that φx xφ1 3x. p 212, #36 The whole point here is that every element of Z Z can be written as a Z- linear combination of 3, 2 and 2, 1. This is because, given any u, v Z Z, the equation x3, 2 + y2, 1 u, v is the same as the vector equation 3 2 u x + y 2 1 v

7 which is the same as the matrix equation 3 2 x 2 1 y u v and the latter has the solution 1 x 3 2 u y 2 1 v u v u + 2v 2u 3v which is a vector with integer entries since u, v Z. From this computation it follows that φu, v φx3, 2 + y2, 1 xφ3, 2 + yφ2, 1 u + 2va + 2u 3vb. In particular φ4, a b 4a 4b. p 213, #52 We will use the one-step subgroup test to prove that H is indeed a subgroup of G. First of all, H since αe e βe implies that e H. Now, if a, b H then αab 1 αaαb 1 αaαb 1 βaβb 1 βaβb 1 βab 1 implying that ab 1 H. Therefore H is a subgroup of G.

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