A Homomorphism from the Additive Group of Integers to Itself

Problem 163

Let $\Z$ be the additive group of integers. Let $f: \Z \to \Z$ be a group homomorphism.
Then show that there exists an integer $a$ such that
\[f(n)=an\]
for any integer $n$.

Let us first recall the definition of a group homomorphism.
A group homomorphism from a group $G$ to a group $H$ is a map $f:G \to H$ such that we have
\[f(gg’)=f(g)f(g’)\]
for any elements $g, g\in G$.

If the group operations for groups $G$ and $H$ are written additively, then a group homomorphism $f:G\to H$ is a map such that
\[f(g+g’)=f(g)+f(g’)\]
for any elements $g, g’ \in G$.

Here is a hint for the problem.
For any integer $n$, write it as
\[n=1+1+\cdots+1\]
and compute $f(n)$ using the property of a homomorphism.

Proof.

Let us put $a:=f(1)\in \Z$. Then for any integer $n$, writing
\[n=1+1+\cdots+1,\]
we have
\begin{align*}
f(n)&=f(1+1+\cdots+1)\\
&=f(1)+f(1)+\cdots+f(1) \quad \text{ since } f \text{ is a homomorphism}\\
&=a+a+\cdots+a\\
&=an.
\end{align*}
Thus we have $f(n)=an$ with $a=f(1)\in \Z$ as required.

Injective Group Homomorphism that does not have Inverse Homomorphism
Let $A=B=\Z$ be the additive group of integers.
Define a map $\phi: A\to B$ by sending $n$ to $2n$ for any integer $n\in A$.
(a) Prove that $\phi$ is a group homomorphism.
(b) Prove that $\phi$ is injective.
(c) Prove that there does not exist a group homomorphism $\psi:B […]

Abelian Groups and Surjective Group Homomorphism
Let $G, G'$ be groups. Suppose that we have a surjective group homomorphism $f:G\to G'$.
Show that if $G$ is an abelian group, then so is $G'$.
Definitions.
Recall the relevant definitions.
A group homomorphism $f:G\to G'$ is a map from $G$ to $G'$ […]

A Group is Abelian if and only if Squaring is a Group Homomorphism
Let $G$ be a group and define a map $f:G\to G$ by $f(a)=a^2$ for each $a\in G$.
Then prove that $G$ is an abelian group if and only if the map $f$ is a group homomorphism.
Proof.
$(\implies)$ If $G$ is an abelian group, then $f$ is a homomorphism.
Suppose that […]

A Group Homomorphism and an Abelian Group
Let $G$ be a group. Define a map $f:G \to G$ by sending each element $g \in G$ to its inverse $g^{-1} \in G$.
Show that $G$ is an abelian group if and only if the map $f: G\to G$ is a group homomorphism.
Proof.
$(\implies)$ If $G$ is an abelian group, then $f$ […]

Abelian Normal subgroup, Quotient Group, and Automorphism Group
Let $G$ be a finite group and let $N$ be a normal abelian subgroup of $G$.
Let $\Aut(N)$ be the group of automorphisms of $G$.
Suppose that the orders of groups $G/N$ and $\Aut(N)$ are relatively prime.
Then prove that $N$ is contained in the center of […]

Surjective Group Homomorphism to $\Z$ and Direct Product of Abelian Groups
Let $G$ be an abelian group and let $f: G\to \Z$ be a surjective group homomorphism.
Prove that we have an isomorphism of groups:
\[G \cong \ker(f)\times \Z.\]
Proof.
Since $f:G\to \Z$ is surjective, there exists an element $a\in G$ such […]

Normal Subgroups, Isomorphic Quotients, But Not Isomorphic
Let $G$ be a group. Suppose that $H_1, H_2, N_1, N_2$ are all normal subgroup of $G$, $H_1 \lhd N_2$, and $H_2 \lhd N_2$.
Suppose also that $N_1/H_1$ is isomorphic to $N_2/H_2$. Then prove or disprove that $N_1$ is isomorphic to $N_2$.
Proof.
We give a […]