Remember, if you have trouble finding the command for a math symbol you want to use, try googling it, email me, or try looking in this Short Math Guide for $\LaTeX{}$.
Let $p$ be a prime. If $G$ is a group of order $p$, then $G\cong \mathbb{Z}_p$. (Theorem 5.21.)
If $m$ and $n$ are relatively prime, then $\mathbb{Z}_m \times \mathbb{Z}_n$ is cyclic. (Only one direction of Theorem 6.15.)
Let $G$ be a group, and let $H\trianglelefteq G$. If $G$ is abelian, then $G/H$ is abelian. (Theorem 6.40)