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## Section8.4Exercises

###### 1

Let $G$ be the internal direct product of subgroups $H$ and $K\text{.}$ Show that the map $\phi : G \rightarrow H \times K$ defined by $\phi(g) = (h,k)$ for $g =hk\text{,}$ where $h \in H$ and $k \in K\text{,}$ is one-to-one and onto.

###### 2

Let $G$ and $H$ be isomorphic groups. If $G$ has a subgroup of order $n\text{,}$ prove that $H$ must also have a subgroup of order $n\text{.}$

###### 3

If $G \cong \overline{G}$ and $H \cong \overline{H}\text{,}$ show that $G \times H \cong \overline{G} \times \overline{H}\text{.}$

###### 4

Prove that $G \times H$ is isomorphic to $H \times G\text{.}$

###### 5

Let $n_1, \ldots, n_k$ be positive integers. Show that

\begin{equation*} \prod_{i=1}^k {\mathbb Z}_{n_i} \cong {\mathbb Z}_{n_1 \cdots n_k} \end{equation*}

if and only if $\gcd( n_i, n_j) =1$ for $i \neq j\text{.}$

###### 6

Prove that $A \times B$ is abelian if and only if $A$ and $B$ are abelian.

###### 7

If $G$ is the internal direct product of $H_1, H_2, \ldots, H_n\text{,}$ prove that $G$ is isomorphic to $\prod_i H_i\text{.}$

###### 8

Let $H_1$ and $H_2$ be subgroups of $G_1$ and $G_2\text{,}$ respectively. Prove that $H_1 \times H_2$ is a subgroup of $G_1 \times G_2\text{.}$

###### 9

Let $m, n \in {\mathbb Z}\text{.}$ Prove that $\langle m,n \rangle = \langle d \rangle$ if and only if $d = \gcd(m,n)\text{.}$

###### 10

Let $m, n \in {\mathbb Z}\text{.}$ Prove that $\langle m \rangle \cap \langle n \rangle = \langle l \rangle$ if and only if $l = \lcm(m,n)\text{.}$