# Math 581 Problem Set 8 Solutions

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1 Math 581 Problem Set 8 Solutions 1. Prove that a group G is abelian if and only if the function ϕ : G G given by ϕ(g) g 1 is a homomorphism of groups. In this case show that ϕ is an isomorphism. Proof: First suppose that G is abelian. Let g h G. Then we have ϕ(gh) (gh) 1 h 1 g 1 g 1 h 1 ϕ(g)ϕ(h) where we used G is abelian to conclude h 1 g 1 g 1 h 1. Thus ϕ is a homomorphism. Now suppose ϕ is a homomorphism. Let g h G. Then we have ϕ(g 1 h 1 ) ϕ(g 1 )ϕ(h 1 ) i.e. (g 1 h 1 ) 1 gh. Thus we have hg gh. Since g and h were arbitrary this shows G is abelian. To see ϕ is an isomorphism we just need to show it is bijective. Let g G. Then ϕ(g 1 ) g and so ϕ is surjective. Suppose ϕ(g) e G. Then g 1 e G i.e. g e G. Thus ϕ is injective as well. 2. Let ϕ : G H be a homomorphism of groups. (a) Let G 1 be a subgroup of G. Prove that ϕ(g 1 ) is a subgroup of H. In particular this shows that ϕ(g) is a subgroup of H. Proof: Recall that ϕ(e G ) e H. Since G 1 is a subgroup e G G 1 and thus e H ϕ(g 1 ) so it is a nonempty set. Let h 1 h 2 ϕ(g 1 ). There exists g 1 and g 2 in G 1 such that ϕ(g i ) h i for i 1 2. We have h 1 h 2 ϕ(g 1 )ϕ(g 2 ) ϕ(g 1 g 2 ). Thus h 1 h 2 ϕ(g 1 ) and so it the set is closed under multiplication. Finally recall that ϕ(g1 1 ) ϕ(g 1) 1 and so h 1 1 ϕ(g 1 ). Hence ϕ(g 1 ) is a subgroup of H. (b) Let H 1 be a subgroup of H. Prove that the set ϕ 1 (H 1 ) {g G : ϕ(g) H 1 } is a subgroup of G. In particular this shows that ϕ 1 (H) is a subgroup of G. Proof: Note that since H 1 is a subgroup of H we have e H H 1. We know that ϕ(e G ) e H so e G ϕ 1 (H 1 ) and thus it is a nonempty set. Let a b ϕ 1 (H 1 ) i.e. ϕ(a) ϕ(b) H 1. Since H 1 is a subgroup we know ϕ(a)ϕ(b) and ϕ(a) 1 are both in H 1. Using that ϕ is a homormophism we get that ϕ(ab) and ϕ(a 1 ) are both in H 1 i.e. ab ϕ 1 (H 1 ) and a 1 ϕ 1 (H 1 ). Thus it is a subgroup. {( ) } 1 n n 3. (a) Prove that n Z is a group under matrix

2 2 multiplication. Proof: Observe that any matrix in H has determinant 1 and hence is an element of GL 2 (Z). Since we have seen GL 2 (Z) is a group we need only check that H is a subgroup ( ) of this group ( to see it is a) group. ( It is clear that ) m m 1 n n H is nonempty as H. Let and 0 1 m 1 + m be in H. Observe that 1 m m 1 n n m 1 + m and ( ) 1 1 n n ( ) 1 (m + n) (m + n) H m + n 1 + (m + n) 1 + n n 1 ( n) ( n) H. n 1 n n 1 + ( n) Thus H is a subgroup of GL 2 (Z) and hence is a group itself. (b) Prove that H Z. ( ) 1 n n Proof: Define ϕ : H Z by n. The fact that we have 1 m m 1 n n m 1 + m ( ) 1 (m + n) (m + n) H m + n 1 + (m + n) makes it clear that ϕ is in fact a homomorphism. ( It is also clear ) that ϕ is 1 n n a surjective map. To see it is injective suppose maps to 0. 1 n n 1 0 Then we must have n 0 and so e 0 1 H. Thus the kernel is just the identity element and hence the map is injective. {} List all the distinct left cosets of in S What is [S 3 : H]? Is H a normal subgroup of S 3?

3 3 There are 3 distinct cosets of H they are ( ) {} e {} {} Since there are 3 distinct cosets we know [S 3 : H] 3. To see that H is in fact not a normal subgroup just observe that / H This is a series of finite group questions. They are not necessarily related. (a) A group has fewer then 100 elements and subgroups of orders 10 and 25. What is the order of G? We know by Lagrange s theorem that 10 G and 25 G. Thus the least common multiple of 10 and 25 namely 50 divides G. However since G < 100 there are no other common multiples of 10 and 25 that could be G. Thus G 50. (b) If H and K are subgroups of a finite group G prove that H K is a common divisor of H and K. Recall we proved in class that H K is a subgroup of H and K. Therefore Lagrange s theorem gives H K H and H K K. (c) If G is a group with more then 1 element and G has no proper subgroups prove that G is isomorphic to Z/pZ for some prime p. Proof: Let a G be such that a e G (since we know G > 1 we can choose such an a). Consider the subgroup a. Since G has no proper subgroups it must be that G a. Thus G is a cyclic group and hence isomorphic to Z/nZ for some positive integer n. However we proved in class that for any proper divisor m of n the set a m is a proper subgroup of a. Since there can be no such subgroups there can be no such proper divisor. Thus n is

4 4 prime and so G Z/pZ for some prime p. (d) If p and q are primes show that every proper subgroup of a group of order pq is cyclic. Proof: By Lagrange s theorem every proper subgroup must be of order 1 p or q. If it is of order 1 then clearly it is just e G. Otherwise it is a group of prime order and hence is isomorphic to Z/pZ or Z/qZ both of which are cyclic. 6. Let a G be fixed and define ϕ : G G by ϕ(x) axa 1. Prove that ϕ is a homomorphism. Under what conditions is ϕ an isomorphism? Proof: Let g h G. Observe that ϕ(gh) agha 1 age G ha 1 aga 1 aha 1 ϕ(g)ϕ(h). Thus ϕ is a homomorphism. To see that ϕ is surjective observe that ϕ(a 1 ga) g. Suppose that g ker ϕ. Thus ϕ(g) e G i.e. aga 1 e G. However solving this for g we obtain g e G. Thus ϕ is injective. Hence ϕ is always an isomorphism. 7. Show that ϕ : R C ϕ(t) cos(2πt) + i sin(2πt) is a homomorphism. What are its kernel and image? Proof: Observe we can write ϕ(t) e 2πit. Let s t R. Then we have ϕ(s + t) e 2πi(s+t) e 2πis e 2πit ϕ(s)ϕ(t). Thus ϕ is a homomorphism. The image of ϕ is precisely the set of complex numbers that can be written in the form e 2πit for some t R. In Math 580 we saw this is the circle of radius 1 in the complex plane. The kernel consists of those t such that e 2πit 1. This is the set of real numbers so that cos(2πt) 1 but sin(2πt) 0 i.e. it is the integers. Thus ker ϕ Z.

5 5 8. Consider the subgroup N Z of the additive group Q. (a) What does it mean for r s(mod N) i.e. for the cosets r +N s+n? It means that r s Z. (b) Show that for m n Z one has m + N n + N. This is clear since m n Z. (c) N? N {r Q : r 1 2 Z} {0 ±1 2 ±1 ±3 2 ±2 ± } (d) How many distinct cosets are there? There are infinitely many distinct cosets. For example the cosets 1 p + Z are all distinct for each prime p. To see this suppose 1 p + Z 1 q + Z where p and q are primes. This means that 1 p 1 q p q Z i.e. qp Z. However since p q this is not 0 and we have pq > p q so this is not an integer. Since there are infinitely many primes there are infinitely many cosets. 9. Consider the additive group G Z/3Z Z/6Z. Set N (1 2). (a) What is N? What about [G : N]? Note that N {(1 2) (2 4) (3 6) (0 0)}. Thus N 3. Since G 18 we have [G : N] 18/3 6. (b) Compute the distinct left cosets that comprise G/N. Note that since we know there are 6 left cosets once we find 6 distinct cosets we are done. Any other left coset must then be one of the 6 we ve found.

6 6 Thus we have (0 0) + N {(0 0) (1 2) (2 4)} (1 0) + N {(1 0) (2 2) (0 4)} (2 0) + N {(2 0) (0 2) (1 4)} (0 1) + N {(0 1) (1 3) (2 5)} (0 3) + N {(0 3) (1 5) (2 1)} (0 5) + N {(0 5) (1 1) (2 3)} (c) Prove that N is a normal subgroup of G. Proof: Since G is an abelian group all the subgroups are normal. (d) Write out an addition table for G/N. Recall that [(a b) + N] + [(c d) + N] (a + c b + d) + N. Using this we have: + (00) + N (10) + N (20) + N (01) + N (03) + N (05) + N (00) + N (00) + N (10) + N (20) + N (01) + N (03) + N (05) + N (10) + N (10) + N (20) + N (00) + N (05) + N (01) + N (03) + N (20) + N (20) + N (00) + N (10) + N (03) + N (05) + N (01) + N (01) + N (01) + N (05) + N (03) + N (05) + N (10) + N (00) + N (03) + N (03) + N (01) + N (05) + N (10) + N (00) + N (20) + N (05) + N (05) + N (03) + N (01) + N (00) + N (20) + N (10) + N Note we have used for example that (1 4) + N (2 0) + N when filling out the table. It is important to only have elements of the group G/N in the table. (e) What familiar group is G/N? We stated in class (but did not prove) that any group of order 6 is either Z/6Z or S 3. Since our group has order 6 and is abelian it must be isomorphic to Z/6Z.

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