Let $G$ be a group. Suppose that $G$ has at most two nontrivial subgroups. Show that $G$ is cyclic.
Can anyone help me please to solve the problem?
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If $G$ has no nontrivial subgroups, it is clearly cyclic. If $G$ has exactly one nontrivial subgroup $H$, consider the subgroup generated by a nonidentity element $g\in G\setminus H$.
Now suppose that $H$ and $K$ are the only nontrivial subgroups of $G$. Recall that a group is never the union of two proper subgroups. So pick some element $g\in G\setminus (H\cup K)$. What must $\langle g\rangle$ be?