Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 7.3 Exercise 7.3.3
Find all homomorphic images of $\mathbb{Z}$.
Solution: Recall that every additive subgroup of $\mathbb{Z}$ is cyclic, and moreover the additive subgroups of $\mathbb{Z}$ are precisely $\langle k \rangle$ for each $k \in \mathbb{Z}$.
Note that for each $ka \in \langle k \rangle$ and $m \in \mathbb{Z}$, $mka = kma \in \langle k \rangle$ and $kam \in \langle k \rangle$, so that $\langle k \rangle$ is a two sided ideal of $\mathbb{Z}$.
Thus the ideals of $\mathbb{Z}$ are precisely of the form $\langle k \rangle$ for each $k \in \mathbb{Z}$. By the First Isomorphism Theorem for rings, the homomorphic images of $\mathbb{Z}$ are precisely $\mathbb{Z}/\langle k \rangle$ for each $k \in \mathbb{Z}$.
