2. Let n 2 3, and G D2n e,r,r2,... ,r"-1,s, sr, sr2,..., sr-'), the dihedral group with 2n ele- 3...
Problem 3.[10 points.] Let D. be the dihedral group of order 2n. Let H = (r) be the subgroup of D, consisting of all rotations. Prove that every subgroup of H is normal in D Solution:
Let De be the dihedral group of order 12. In other words, Do = {e,r,r2, m3, 74, 75, s, rs, rs, rºs, r*s, r® s} where p6 = 92 = e and sr = r-1s. a. Is H = {1, s, sr, sr2} a subgroup of Do? Why or why not? b. Is K = {1, 8, 73, r3s} a subgroup of Do? Why or why not?
10. Let G = D. be the dihedral group on the octagon and let N = (r) be the subgroup of G generated by r4. (a) Prove that N is a normal subgroup of G. (b) If G =D3/N, find G. (c) Using the bar notation for cosets, show that G = {e, F, 2, 3, 5, 87, 82, 83}. Hint: Show that the RHS consists of distinct elements and then use part (b). (d) Prove that G-D4. Hint: It...