__Theorem 1__**:** Every subgroup of a cyclic group is cyclic.

**Proof:** Let be a cyclic group generated by . Let be a subgroup of . Now every element of , hence also of , has the form , being an integer. Let be the smallest possible integer such that . We claim that . For this it is sufficient to show that , then for then . Now, if does not divide , then there exist integers and such that

Then

or

Since , it follows that and hence its inverse .

But by supposition. Then from the above result it follows that , contrary to the choice of since was assumed to be the least positive integer such that . Therefore and so . But then

Thus every element of is of the form . Hence .

__Theorem 2__**:**Every subgroup of an infinite cyclic group is infinite.

**Proof: **Let be infinite cyclic group. Let be a subgroup of . Then by the preceding theorem, where is the least positive integer such that . Now suppose, if possible, that is finite.

This implies that for some .

It follows that is of finite order and this in turn implies that is finite, contrary to the hypothesis. Hence must be an infinite cyclic subgroup of .