# 1396 I Isomorphisms10. Let G be a group. Prove that the mapping a(g) = g1 for all g in Gis an automorphism if and only if G is Abelian.11. If g and h are elements from a group, prove that h12. Find two groups G and H such that G H, but Aut(G) Aut(H).13. Prove the assertion in Example 12 that the inner automorphismsand o, of Di are distinct.PRa14. Find Aut(Z)15. If G is a group, prove that Aut(G) and Inn(G)16. If a group G is isomorphic to H, prove that Aut(G) is isomorphic toAut(H).are groups.17. Suppose belongs to Aut(Z,) and a is relatively prime to n.If d(a) b, determine a formula for p(x).18. Let H be the subgroup of all rotations in D. and let d be an auto-morphism of D. Prove that (H) = H. (In words, an automor-phism of D, carries rotations to rotations.)19. Let H {BES,IB(1) 1} and Kthat H is isomorphic to K. Is the same true if S, is replaced by Swhere n 3?20. Show that Z has infinitely many subgroups isomorphic to Z.п11nn2}. Prove{BE SIB(2)п>21. Let n be an even integer greater than 2 and let o be an automor-phism of D. Determine (R, 80).22. Let be an automorphism of a group G. Prove that H {x EG(x) =x} is a subgroup of G23. Give an example of a cyclic group of smallest order that contains asubgroup isomorphic to Z12 and a subgroup isomorphic to Z20 Noneed to prove anything, but explain your reasoning.24. Suppose that d: Z20Z20 is an automorphism and (5) 5. Whatare the possibilities for d(x)?25. Identify a group G that has subgroups isomorphic to Z for all pos-itive integers n.26. Prove that the mapping from U(16) to itself given by x -x3 is anautomorphism. What about xx and x -x? Generalize.27. Let r E U(n). Prove that the mapping a: Z> Z defined by a(s) =sr mod n for all s in Z, is an automorphism of Z (This exercise isreferred to in this chapter.)n'28. The group aEZis isomorphic to what familiargroup? What if Z is replaced by R?U

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22.

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Problem 22.

To prove that H is a subgroup of G, show that...

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