Isomorphisms, Products and Decomposition
Isomorphisms
See this video and these notes.
Definition: Let $G$ and $H$ be groups. An isomorphism $f:G\to H$ is a bijective map with the property that, for all $a$,$b$ in $G$, we have $f(ab)=f(a)f(b)$. If $G$ and $H$ are two groups and there exists an isomorphism $f:G\to H$, we say that $G$ and $H$ are isomorphic.
Examples of isomorphic and nonisomorphic groups

The additive group of reals $\mathbb{R}$ and the multiplicative group of positive reals \(\mathbb{R}_{+}^{\star}\) are isomorphic. One isomorphism $f:\mathbb{R}\to\mathbb{R}_{+}^{\star}$ is the exponential map $f(x)=e^{x}$.

\(\mathbb{Z}_{4}\) is isomorphic to the subgroup \(\langle i \rangle\subset\mathbb{C}^{\star}\) via the map sending $f(x)=i^{x}$.

If $a\in\mathbb{Q}^{\star}$ and $a>1$, the subset \(\langle a \rangle=\{a^{n} : n\in\mathbb{Z}\}\) is isomorphic to $\mathbb{Z}$ via the map $f(n)=a^{n}$.

\(\mathbb{Z}_{n}\) and \(\mathbb{Z}_{m}\) are not isomorphic when $n\not=m$ because they have different numbers of elements.

\(\mathbb{Z}_{6}\) and \(S_{3}\) both have six elements, but they are not isomorphic because one is abelian and one is not.
Properties of isomorphisms.
See this video and these notes.
Theorem: Let $f:G\to H$ be an isomorphism between $G$ and $H$. Then:
 $f^{1}$ is an isomorphism from $H$ to $G$.
 $G$ and $H$ have the same number of elements.
 if one of $G$ or $H$ is abelian, so is the other.
 if one of $G$ or $H$ is cyclic, so is the other.
 if one of $G$ or $H$ has a subgroup of order $n$, so does the other.
Some classification results
See this video and these notes.
Theorem: The isomorphism of groups is an equivalence relation among groups.
 Any infinite cyclic group is isomorphic to $\mathbb{Z}$.
 Any finite cyclic group of order $n$ is isomorphic to $\mathbb{Z}_{n}$.
 Any group of order $p$, where $p$ is prime, is isomorphic to $\mathbb{Z}_{p}$.
Cayleyâ€™s Theorem
See this video and these notes.
Theorem: Any group $G$ is isomorphic to a group of permutations. If $G$ is finite, then $G$ is isomorphic to a subgroup of $S_{n}$ for some positive integer $n$.
Direct Products
See this video and these notes.
If $G$ and $H$ are groups, the cartesian product $G\times H$ can be made into a group by componentwise multiplication.
Theorem: The group \(\mathbb{Z}_{m}\times\mathbb{Z}_{n}\) is isomorphic to \(\mathbb{Z}_{mn}\) if and only if \(\mathop{gcd}(m,n)=1\).
Corollary: (Decomposition theorem for \(\mathbb{Z}_{n}\)) Suppose that \(n=p_{1}^{e_{1}}\cdots p_{k}^{e_{k}}\) is the prime factorization of $n$. Then \(\mathbb{Z}_{n} = \mathbb{Z}_{p_{1}^{e_{1}}}\times\cdots\times\mathbb{Z}_{p_{k}^{e_{k}}}.\)
Internal direct products
See this video and these notes.
Definition: Let $G$ be a group and let $H$ and $K$ be two subgroups such that $H\cap K={e}$, $hk=kh$ for all $k\in K$ and $h\in H$; and every element $g\in G$ can be written $hk$ for some $h\in H$ and some $k\in K$. Then $G$ is called the internal direct product of $H$ and $K$.
If $G$ is the internal direct product of $H$ and $K$ then $G$ is isomorphic to $H\times K$.
More generally, suppose that $H_1,\ldots, H_n$ are subgroups of a group $G$ which:
 generate $G$;
 are such that the only element in common between any two $H_{i}$ is the identity;
 mutually commute (so that $h_ih_j=h_jh_i$ for any $h_i\in H_{i}$ and $h_j\in H_{j}$)
Then $G$ is the internal direct product of the $H_{i}$ and is isomorphic to their direct product.
Examples of internal direct products

The dihedral group of the hexagon $D_{6}$ is the internal direct product of two subgroups $H$ and $K$, where \(H=\{e, r^{3}\}\) and \(K = \{e, r^{2}, r^{4}, s, r^{2}s, r^{4}s\}.\)

$S_{3}$ is not an internal direct product of any two of its nontrivial subgroups.