# Algebra I: Final 2012 June 22, 2012

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1 1 Algebra I: Final 2012 June 22, 2012 Quote the following when necessary. A. Subgroup H of a group G: H G = H G, xy H and x 1 H for all x, y H. B. Order of an Element: Let g be an element of a group G. Then g = {g n n Z} is a subgroup of G. If there is a positive integer m such that g m = e, where e is the identity element of G, g = min{m g m = e, m N} and g = g. Moreover, for any integer n, g divides n if and only if g n = e. C. Lagrange s Theorem: If H is a subgroup of a finite group G, then G = G : H H. D. Normal Subgroup: A subgroup H of a group G is normal if ghg 1 = H for all g G. If H is a normal subgroup of G, then G/H becomes a group with respect to the binary operation (gh)(g H) = gg H. E. Direct Product: If gcd{m, n} = 1, then Z mn Z m Z n and U(mn) U(m) U(n). F. Kernel: If φ : G G is a group homomorphism, Ker(φ) = {x G φ(x) = e G }, where e G is the identity element of G. G. Sylow s Theorem: For a finite group G and a prime p, let Syl p (G) denote the set of Sylow p-subgroups of G. Then Syl p (G). Let P Syl p (G). Then Syl p (G) = G : N(P ) 1 (mod p), where N(P ) = {x G xp x 1 = P }. 1. Let N be a subgroup of a group G such that xg = gx for all x N and g G. (10 pts) (a) Show that N G. (b) Show that if G/N is cyclic, then G is Abelian.

2 2 2. Let H and K be subgroups of a group G. Show the following. (25 pts) (a) For x G, xh = H if and only if x H. (b) HH 1 = H. (c) If xhx 1 H for all x G and h H, then H is a normal subgroup of G. (d) If H is a normal subgroup of G, then HK is a subgroup of G. (e) If α : G G is a group homomorphism, then α(x) x for every x G of finite order.

3 3 3. Let G be a finite group and H a subgroup of G such that G : H = n. For each g G let α g : G/H G/H (xh gxh). (20 pts) (a) Show that α g Sym(G/H), i.e, α g is a permutation on G/H. (b) Show that φ : G Sym(G/H) (g α g ) is a group homomorphism. (c) Show that Kerφ = xhx 1. x G (Hint: gxh = xh gxhx 1 = xhx 1 and xhx 1 G.) (d) G has a normal subgroup N such that N H and that G/N n!.

4 4 4. Answer the following questions on Abelian groups of order 80 = (20 pts) (a) Using the Fundamental Theorem of Finite Abelian Groups and list all non-isomorphic Abelian groups of order 80 and give a brief explanation. (b) List all Abelian groups of order 80 in your list above that have exactly three elements of order 2. Give your reason. (c) Determine whether or not U(200) U(220). Give your reason.

5 5 5. Let G be a group of order 60, P a Sylow 2-subgroup, Q a Sylow 3-subgroup and R a Sylow 5-subgroup of G. Suppose that {e} and G are the only normal subgroups of G. Prove the following. (25 pts) (a) Show that P is Abelian. (b) Show that Q and R are cyclic. (c) Show that there are exactly 6 Sylow 5-subgroups and N(R) = 10. (d) Let H be a proper subgroup of G containing P. Then H = 4 or 12. (e) G A 5. (Hint: Show that G has a subgroup of order 12 and use 3.) Please write your message: Comments on group theory. Suggestions for improvements of this course. Write on the back of this sheet is also welcome.

6 1 Algebra I: Solutions to Final 2012 June 22, Let N be a subgroup of a group G such that xg = gx for all x N and g G. (10 pts) (a) Show that N G. Soln. Let g G. Then by assumption, Hence N G. gng 1 = {gxg 1 x N} = {x x N} = N. (b) Show that if G/N is cyclic, then G is Abelian. Soln. Let G/N = gn = {(gn) n n Z} = {g n N n Z}. Let a, b G. Then there exist n, m Z such that a g n N and b g m N. Hence there exist x, y N such that a = g n x, b = g m y. Now using assumption, we have Thus G is Abelian. ab = g n xg m y = g n g m xy = g m g n yx = g m yg n x = ba. 2. Let H and K be subgroups of a group G. Show the following. (25 pts) (a) For x G, xh = H if and only if x H. Soln. Suppose xh = H. Since H is a subgroup, e H. Hence x = xe xh = H. Thus x H. Conversely, if x H, then since H is a subgroup, Therefore xh = H. (b) HH 1 = H. Soln. Since H is a subgroup, xh HH H = eh = xx 1 H xhh xh. H = He 1 HH 1 H. Therefore H = HH 1. (One can use (a) as well. HH 1 = h H Hh 1 = h H H = H.) (c) If xhx 1 H for all x G and h H, then H is a normal subgroup of G. Soln. By assumption, xhx 1 H for every x G. Since x 1 G, x 1 Hx = x 1 H(x 1 ) 1 H. Therefore xhx 1 H = x(x 1 Hx)x 1 xhx 1. Thus xhx 1 = H for every x G, and H is a normal subgroup of G. (d) If H is a normal subgroup of G, then HK is a subgroup of G. Soln. Suppose H is a normal subgroup of G. Then e = ee HK and HK. Let x, y HK. Then there exist h, h H and k, k K such that x = hk and y = h k. Since H is normal, h = kh k 1 H and xy = hkh k = h(kh k 1 )kk = hh kk HK. Similarly since h = k 1 h 1 k k 1 H(k 1 ) 1 = H, x 1 = (hk) 1 = k 1 h 1 = (k 1 h 1 k)k 1 = h k 1 HK. Therefore HK is a subgroup of G.

7 2 (e) If α : G G is a group homomorphism, then α(x) x for every x G of finite order. Soln. Firstly, since α(e) = α(ee) = α(e)α(e), we have e = α(e) by multiplying α(e) 1. Let n = x. Then x n = e. So e = α(e) = α(x n ) = α(x) n. Thus α(x) n by B. 3. Let G be a finite group and H a subgroup of G such that G : H = n. For each g G let α g : G/H G/H (xh gxh). (20 pts) (a) Show that α g Sym(G/H), i.e, α g is a permutation on G/H. Soln. Since α g (xh) = α g (yh) implies gxh = gyh and xh = yh, α g is one-toone. Since G/H is a finite set, α g is a bijection and α g Sym(G/H). (b) Show that φ : G Sym(G/H) (g α g ) is a group homomorphism. Soln. Since φ(gg ) = α gg and φ(g)φ(g ) = α g α g, we need to show α gg = α g α g in Sym(G/H). This holds as α gg (xh) = gg xh = g(g xh) = α g (α g (xh)) = (α g α g )(xh). Thus φ is a group homemorphism. (c) Show that Kerφ = xhx 1. x G (Hint: gxh = xh gxhx 1 = xhx 1 and xhx 1 G.) Soln. g Kerφ if and only if α g = id if and only if gxh = xh for all x G. Since gxh = xh gxhx 1 = xhx 1 and xhx 1 is a subgroup of G, we have g xhx 1. Thus g Kerφ ( x G)[g xhx 1 ] g xhx 1. Therefore we have the assertion. (d) G has a normal subgroup N such that N H and that G/N n!. Soln. Since G : H = n, Sym(G/H) S n. Let N = Kerφ H. By first isomorphism theorem, G/N is isomorphic to a subgroup of Sym(G/H) and Sym(G/H) = n!. Therefore G/N n!. 4. Answer the following questions on Abelian groups of order 80 = (20 pts) (a) Using the Fundamental Theorem of Finite Abelian Groups and list all non-isomorphic Abelian groups of order 80 and give a brief explanation. Soln. Since 4 = 4, 1 + 3, 2 + 2, , , there are five isomorphism classes of Abelian groups of order 80 = They are x G Z 80, Z 2 Z 40, Z 4 Z 20, Z 2 Z 2 Z 20, Z 2 Z 2 Z 2 Z 10. (b) List all Abelian groups of order 80 in your list above that have exactly three elements of order 2. Give your reason. Soln. For each of the Abelian groups above, elements of order 2 are in Z 2, Z 2 Z 2, Z 2 Z 2, Z 2 Z 2 Z 2, Z 2 Z 2 Z 2 Z 2. Hence Z 2 Z 40 and Z 4 Z 20 are those having three elements of order 2.

8 3 (c) Determine whether or not U(200) U(220). Give your reason. Soln. U(200) = U( ) U(2 3 ) U(5 2 ) Z 2 Z 2 Z 20 as all nonidentity elements of U(2 3 ) are of order 2, and U(5 2 ) is generated by 2. (Since the order of U(5 2 ) is 20, it suffices to show the existence of an element of order divisible by (mod 25). So the order of 7 is four.) U(220) = U( ) U(2 2 ) U(5) U(11) Z 2 Z 4 Z 10 Z 2 Z 2 Z 20. Hence these groups are isomorphic. 5. Let G be a group of order 60, P a Sylow 2-subgroup, Q a Sylow 3-subgroup and R a Sylow 5-subgroup of G. Suppose that {e} and G are the only normal subgroups of G. Prove the following. (25 pts) (a) Show that P is Abelian. Soln. P is of order 4. Let a P be a nonidentity element. Then a = 2, 4. If there is an element of order 4, P is cyclic and P is Abelian. Hence we may assume that x 2 = e for every x P. For x, y P, xy = xy(yx) 2 = (x(yy)x)yx = yx and P is Abelian. (b) Show that Q and R are cyclic. Soln. Q and R are of prime order. Let x Q and y R be nonidentity elements. Then 1 x 3 and 1 y 5, and x = 3, y = 5, Therefore Q = x and R = y are both cyclic. (c) Show that there are exactly 6 Sylow 5-subgroups and N(R) = 10. Soln. Syl 5 (G) 1 (mod 5) are divisors of G = 60, we have Syl 5 (G) = 1 or 6. If Syl 5 (G) = 1, R is normal. This contradicts our assumption. Hence 6 = Syl 5 (G) = G : N(R), and N(R) = 10 by C. (d) Let H be a proper subgroup of G containing P. Then H = 4 or 12. Soln. Since 4 H, we need to show that H = 20. Suppose H = 20 and R H. Then Syl 5 (H) = 1. Thus R H, and N(R) is divisible by 4. This contradicts (c). (e) G A 5. (Hint: Show that G has a subgroup of order 12 and use 3.) Soln. Suppose N(P ) = 4, i.e., N(P ) = P. Let g G P. Suppose e z P gp g 1. Then C(z) contains both P and gp g 1 and so C(z) is a subgroup properly containing P. This contradicts our assumption. Hence P gp g 1 = {e} for all g P. This is absurd as g G gp g 1 must contain 46 elements, while there are 24 elements of order 5. Thus G has a subgroup H of order 12. Since G : H = 5, G is isomorphic to a subgroup G of S 5. Since A 5 S 5, G A 5 G and G = A 5. Note that every Sylow 5-subgoup of S 5 is in A 5 and hence G A 5 {e}. This proves the assertion.

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