\begin{definition}[Definition:Normal Series/Factor Group]
Let $G$ be a group whose identity is $e$.
Let $\sequence {G_i}_{i \mathop \in \closedint 0 n}$ be a normal series for $G$:
:$\sequence {G_i}_{i \mathop \in \closedint 0 n} = \tuple {\set e = G_0 \lhd G_1 \lhd \cdots \lhd G_{n - 1} \lhd G_n = G}$
The '''factor groups''' of $\sequence {G_i}_{i \mathop \in \closedint 0 n}$:
:$\set e = G_0 \lhd G_1 \lhd \cdots \lhd G_n = G$
are the quotient groups:
:$G_1 / G_0, G_2 / G_1, \ldots, G_i / G_{i - 1}, \ldots, G_n / G_{n-1}$
\end{definition}