The subequations environment provides a convenient way to number equations in a group with a subordinate numbering scheme. For example, supposing that the current equation number is 2.8, write
\begin{equation}\label{first}
a=b+c
\end{equation}
some intervening text
\begin{subequations}\label{grp}
\begin{align}
a&=b+c\label{second}\\
d&=e+f+g\label{third}\\
h&=i+j\label{fourth}
\end{align}
\end{subequations}
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to get
By putting a \label command immediately after \begin{subequations } you can get a
reference to the parent number; \eqref{grp} from the above example would produce (2)
while \eqref {second } would produce (2a).
You can also use the subequations environment to skip an equation number but record it
in a label. This could be used to give an equation number to a Figure or list, for
example
\begin{equation}
ax = y\,,
\end{equation}
with properties
\begin{subequations}
\label{list1}
\end{subequations}
\begin{itemize}
\item[(\ref{list1}.1)] $x$ and $y$ are vectors
\item[(\ref{list1}.2)] $a$ is a scalar
\end{itemize}
\begin{equation}
bx=cy\,.
\end{equation}
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gives
