# Kevin James. MTHSC 412 Section 3.4 Cyclic Groups

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1 MTHSC 412 Section 3.4 Cyclic Groups

2 Definition If G is a cyclic group and G =< a > then a is a generator of G.

3 Definition If G is a cyclic group and G =< a > then a is a generator of G. Example 1 Z is a cyclic group and can be generated by 1 or 1.

4 Definition If G is a cyclic group and G =< a > then a is a generator of G. Example 1 Z is a cyclic group and can be generated by 1 or 1. 2 The group G = {e, (1, 2, 3, 4), (1, 3)(2, 4), (1, 4, 3, 2)} S 4 is cyclic and G =< (1, 2, 3, 4) >.

5 Definition If G is a cyclic group and G =< a > then a is a generator of G. Example 1 Z is a cyclic group and can be generated by 1 or 1. 2 The group G = {e, (1, 2, 3, 4), (1, 3)(2, 4), (1, 4, 3, 2)} S 4 is cyclic and G =< (1, 2, 3, 4) >. Also, G =< (1, 4, 3, 2) >.

6 Definition If G is a cyclic group and G =< a > then a is a generator of G. Example 1 Z is a cyclic group and can be generated by 1 or 1. 2 The group G = {e, (1, 2, 3, 4), (1, 3)(2, 4), (1, 4, 3, 2)} S 4 is cyclic and G =< (1, 2, 3, 4) >. Also, G =< (1, 4, 3, 2) >. Fact If a is a generator of G, then so is a 1 (or a if we are using additive notation).

7 Let a G. If a n e for all n Z, then a p a q for all p q Z and G is infinite.

8 Let a G. If a n e for all n Z, then a p a q for all p q Z and G is infinite. Corollary If G is a finite group and a G, then there exists n N such that a n = e.

9 Let a G. If a n e for all n Z, then a p a q for all p q Z and G is infinite. Corollary If G is a finite group and a G, then there exists n N such that a n = e. Example S 3 is a finite group. For each element σ S 3 find the positive integer n such that σ n = e.

10 Let a G and suppose that a k = e for some k Z. Then there is a smallest positive integer m such that a m = e and 1 < a > has order m and < a >= {a 0 = e = a m, a, a 2,..., a m 1 }. 2 a r = a s if and only if r s (mod m).

11 Definition The order of an element a G is defined by o(a) = < a >.

12 Definition The order of an element a G is defined by o(a) = < a >. Fact If a G and o(a) is finite then o(a) is the least positive integer m such that a m = e.

13 Suppose that G is cyclic and G =< a >. If H G, then either 1 H =< e >, or 2 If H < e >, then H =< a k > where k is the least positive integer such that a k H.

14 Suppose that G is cyclic and G =< a >. If H G, then either 1 H =< e >, or 2 If H < e >, then H =< a k > where k is the least positive integer such that a k H. Corollary Any subgroup of a cyclic group is cyclic.

15 Suppose that G =< a > is cyclic of order n. If m Z and d = (m, n) then < a m >=< a d >.

16 Suppose that G =< a > is cyclic of order n. If m Z and d = (m, n) then < a m >=< a d >. Fact Suppose that G =< a > is cyclic of order n and that d n. Then o(a d ) = < a d > = n/d.

17 Suppose that G =< a > is cyclic of order n. If m Z and d = (m, n) then < a m >=< a d >. Fact Suppose that G =< a > is cyclic of order n and that d n. Then o(a d ) = < a d > = n/d. Corollary Let G =< a > be a cyclic group of order n. The distinct subgroups of G are the groups < a k > where k is a positive divisor of n.

18 Example Suppose that G = Z 10. Note that G =< 1 > is cyclic of order 10. So, the distinct subgroups are: 1 < 0 >= {0} which has order 1. 2 < 5 >= {0, 5} which has order 2. 3 < 2 >= {0, 2, 4, 6, 8} which has order 5, and 4 < 1 >= {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} which has order 10.

19 Example Suppose that G S 6 is the cyclic group generated by (1, 2, 3, 4, 5, 6). That is, G = {e, (1, 2, 3, 4, 5, 6), (1, 3, 5)(2, 4, 6), (1, 4)(2, 5)(3, 6), (1, 5, 3)(2, 6, 4), (1, 6, 5, 4, 3, 2)} The distinct subgroups are:

20 Example Suppose that G S 6 is the cyclic group generated by (1, 2, 3, 4, 5, 6). That is, G = {e, (1, 2, 3, 4, 5, 6), (1, 3, 5)(2, 4, 6), (1, 4)(2, 5)(3, 6), (1, 5, 3)(2, 6, 4), (1, 6, 5, 4, 3, 2)} The distinct subgroups are: 1 < e > which has order 1, 2 < (1, 2, 3, 4, 5, 6) 3 >=< (1, 4)(2, 5)(3, 6) >= {e, (1, 4)(2, 5)(3, 6)} which has order 2, 3 < ((1, 2, 3, 4, 5, 6) 2 >=< (1, 3, 5)(2, 4, 6) >= {e, (1, 3, 5)(2, 4, 6), (1, 5, 3)(2, 6, 4)} which has order 3, and 4 G =< (1, 2, 3, 4, 5, 6) > which has order 6.

21 Let G =< a > be a cyclic group of order n. Then a m is a generator of G if and only if (m, n) = 1.

22 Let G =< a > be a cyclic group of order n. Then a m is a generator of G if and only if (m, n) = 1. Example 1 Suppose that G =< a > is a cyclic group of order 9. Then the generators of G are a, a 2, a 4, a 5, a 7, a 8.

23 Let G =< a > be a cyclic group of order n. Then a m is a generator of G if and only if (m, n) = 1. Example 1 Suppose that G =< a > is a cyclic group of order 9. Then the generators of G are a, a 2, a 4, a 5, a 7, a 8. 2 The generators of Z 10 are 1,3,7, and 9.

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