# union of proper subgroup is proper?

Question:

Prove that a finite group is the union of proper subgroups IFF the group is not cyclic.

Let G be a finite group.

Suppose G is the union of proper subgroups $$b_{i}$$.

This means that there is an element in G that is not in $$b_{1}$$. Iterating this reasoning, we see that there is at least one element in G that is not in the union of the proper subgroups $$b_{i}$$. Then what? This feels like those days where no questions can be solved.

Any hint is appreciated. Thanks in advance.

Edit: If G were cyclic, then, there exists an element, say $$a$$, that generates G. But $$G=\cup _{i=1}^{n}b_{i}$$ implies that $$a$$ generates $$\cup _{i=1}^{n}b_{i}$$ too. Hence, $$\cup _{i=1}^{n}b_{i}$$ is not a proper union of subgroup since every element in G is in $$\cup _{i=1}^{n}b_{i}$$.

Assume $G=\displaystyle\bigcup_{i\in I} H_i$, and let any $x\in G$. Then $x\in H_i$ for some $I$ and so $\langle x\rangle \subset H_i$, and so since $H_i$ is proper, $\langle x\rangle$ is as well: $x$ doesn't generate $G$. This is for any $x\in G$, so that $G$ is not cyclic.

Conversely, assume $G$ is not cyclic. Then $G=\displaystyle\bigcup_{x\in G}\langle x\rangle$ is a union of proper subgroups.

• Is the element x necessarily restricted to being in one proper subgroup or can it be contained in the intersection of at least 2 proper subgroup? Commented Apr 15, 2017 at 10:03
• $x$ can be in many subgroups, that doesn't change anything Commented Apr 15, 2017 at 10:55

Let $G$ be a group, finite or infinite. Observe that the following statements are equivalent.

1. $G$ is the union of some proper subgroups.
2. $G$ is the union of all of its proper subgroups.
3. Each element of $G$ belongs to some proper subgroup of $G.$
4. For each element $g\in G,$ $\langle g\rangle$ is a proper subgroup of $G.$
5. There is no element $g\in G$ such that $\langle g\rangle=G.$
6. $G$ is not cyclic.