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### Problem 8

Let $\phi : G \rightarrow G'$ be an isomorphism of a group $\langle G, * \rangle$ with the group $\langle G',*' \rangle$. Prove: If $H$ is a subgroup of $G$, then $\phi [H] = \{ \phi (h) \ h\in H \}$ is a subgroup of $G'$. That is, an isomorphism carries subgroups into subgroups.

**Solution**

(closed)

Let $a,b \in H$ such that $\phi (a) , \phi (b) \in \phi [H]$.

Then $(a*b) \in H$ since $H \leq G$, and is therefore closed.

Since $\phi$ is an isomorphism $\phi (a) *' \phi (b) = \phi (a*b) \in \phi [H]$, so $\phi [H]$ is closed under $*'$.

(identity)

By Theorem 3.14 since $\phi$ is an isomorphism $e' = \phi (e) \in \phi [H]$.

(inverse)

Let $a \in H$ such that $\phi (a) \in \phi [H]$. Then $a^{-1} \in H$ by definition of a subgroup. It follows that

(1)then,

(2)Therefore, since all conditions hold, $\phi [H] = \{ \phi (h) \ h\in H \}$ is a subgroup of $G'$.