Subequations

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}

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}

gives