# The Abelian Group of Even Integers

Recall from the Abelian Groups page that an Abelian group is a group with the extra property of commutativity on its operation $*$. That is, an Abelian group is a set $G$ paired with a binary operation $* : G \times G \to G$ where:

- For all $a, b \in G$ we have that $(a * b) \in G$ (Closure under $*$).

- For all $a, b, c \in G$ we have that $(a * b) * c = a * (b * c)$ (Associativity of $*$).

- There exists an element $e \in G$ such that $a * e = a$ and $e * a = a$ (The existence of an identity for $*$).

- For all $a \in G$ there exists a $a^{-1} \in G$ such that $a * a^{-1} = e$ and $a^{-1} * a = e$ (The existence of inverses for each element in $G$).

- For all $a, b \in G$ we have that $a * b = b * a$ (Commutativity of $*$).

We will now look at the Abelian group of even integers denoted $2 \mathbb{Z}$.

Let $2\mathbb{Z}$ be the set defined as:

(1)Define the operation $*$ as standard addition so that for all $a, b \in 2\mathbb{Z}$ we have:

(2)We will now check all of the group axioms to verify that $2 \mathbb{Z}$ is a group under $*$. Let $a, b, c \in \mathbb{Z}$. Then $a = 2n_1$, $b = 2n_2$, and $c = 2n_3$ for $n_1, n_2, n_3 \in \mathbb{Z}$. We have that $2 \mathbb{Z}$ is closed under $*$ since:

(3)We also have that $*$ is commutative since:

(4)The identity element for $*$ is the even integer $0 = 0n$ since:

(5)And similarly:

(6)For each $a \in 2\mathbb{Z}$ we have that the inverse of $a$ is $-a = -2n_1 = 2(-n_1) \in 2 \mathbb{Z}$ since:

(7)And similarly:

(8)Therefore $(2\mathbb{Z}, *)$ forms a group. We now only need to show that commutativity for $*$ holds to show that $2 \mathbb{Z}$ is an Abelian group. We have that:

(9)Therefore $(2\mathbb{Z}, *)$ is an Abelian group.