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NOTE that $Z_{n}$ is a cyclic group, hence an abelian group.
For any group, we know by Lagrange's theorem that the order of subgroup divides the order of the group.

For abelian groups, the converse of Lagrange's theorem is also true. For the Abelian group, we know that if $d$ divides the order of the group then there exists a subgroup of size $d.$

The subgroups of $Z_9$ are:

  1. $\{0\},$
  2. $\{0, 3, 6\},$
  3. $Z_{9}.$
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