Subgroups of Additive Group of Integers
Theorem
Let $\struct {\Z, +}$ be the additive group of integers.
Let $n \Z$ be the additive group of integer multiples of $n$.
Every non-trivial subgroup of $\struct {\Z, +}$ has the form $n \Z$.
Proof
First we note that, from Integer Multiples under Addition form Infinite Cyclic Group, $\struct {n \Z, +}$ is an infinite cyclic group.
From Cyclic Group is Abelian, it follows that $\struct {n \Z, +}$ is an infinite abelian group.
Let $H$ be a non-trivial subgroup of $\struct {\Z, +}$.
Because $H$ is non-trivial:
- $\exists m \in \Z: m \in H: m \ne 0$
Because $H$ is itself a group:
- $-m \in H$
So either $m$ or $-m$ is positive and therefore in $\Z_{>0}$.
Thus:
- $H \cap \Z_{>0} \ne \O$
From the Well-Ordering Principle, $H \cap \Z_{>0}$ has a smallest element, which we can call $n$.
It follows from Subgroup of Infinite Cyclic Group is Infinite Cyclic Group that:
- $\forall a \in \Z: a n \in H$
Thus:
- $n \Z \subseteq H$
Aiming for a contradiction, suppose:
- $\exists m \in \Z: m \in H \setminus n \Z$
Then $m \ne 0$, and also $-m \in H \setminus n \Z$.
Assume $m > 0$, otherwise we consider $-m$.
By the Division Theorem:
- $m = q n + r$
If $r = 0$, then $m = q n \in n \Z$, so $0 \le r < n$.
Now this means $r = m - q n \in H$ and $0 \le r < n$.
This would mean $n$ was not the smallest element of $H \cap \Z$.
Hence, by Proof by Contradiction, there can be no such $m \in H \setminus n \Z$.
Thus:
- $H \setminus n \Z = \O$
Thus from Set Difference with Superset is Empty Set:
- $H \subseteq n \Z$
Thus we have $n \Z \subseteq H$ and $H \subseteq n \Z$.
Hence:
- $H = n \Z$
$\blacksquare$
Examples
Even Integers
Let $2 \Z$ denote the set of even integers.
Let $\struct {2 \Z, +}$ be the algebraic structure formed from $2 \Z$ with the operation of integer addition.
Then $\struct {2 \Z, +}$ is a subgroup of the additive group of integers $\struct {\Z, +}$.
Multiples of $4$
Let $4 \Z$ denote the set of integers which are divisible by $4$.
Let $\struct {4 \Z, +}$ be the algebraic structure formed from $4 \Z$ with the operation of integer addition.
Then $\struct {4 \Z, +}$ is a subgroup of the additive group of integers $\struct {\Z, +}$.
Sources
- 1967: George McCarty: Topology: An Introduction with Application to Topological Groups ... (previous) ... (next): Chapter $\text{II}$: Groups: A Little Number Theory
- 1971: Allan Clark: Elements of Abstract Algebra ... (previous) ... (next): Chapter $2$: Subgroups and Cosets: $\S 36$
- 1974: Thomas W. Hungerford: Algebra ... (previous) ... (next): $\S 1.2$