Topoltergeist wrote:The subgroup generated by (1,1) will include the following four elements:
Another way that you could deduce that this group has 4 elements is that it is the product of a 2-cycle in Z/2Z and a 4-cycle in Z/4Z. The least common multiple of 2 and 4 is 4, hence the subgroup generated has order 4.
That being said, use Lagrange's theorem: for a finite group G with a subgroup H, we have |G| = [G:H] |H|. Let G be the group Z/2Z X Z/4Z and let H be the subgroup generated by (1,1). Following through with the computation we obtain the answer:
| Z/2Z X Z/4Z | = |Z/2Z| * |Z/4Z| = 2*4 = 8
|H| = 4
8 = [G:H] 4
[G:H] = 2
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