[Tex/LaTex] Number equations within array

cross-referencingnumberingsubequations

I want to number sub-items of an array I have in order to refer to the right equation and not to the set of equations (SEE the pictures attached):

Figure that should call 1.35a, 1.35b, 135.c, 1.36a, 1.36b and 1.36c (and should have also sublabels):

vv.

Picture that should have sublabels:
vv

I found two similar posts (like Subequations and Sub-numbering equations within array) but they don't apply and I couldn't implement in my example. I tried to replace my "equation" entries for subequations and label each line but it didn't work. What should I try?

An example of the array I have so far is (the example is for 1.34):

\begin{equation}
 \left\{
    \def\arraystretch{1.8}
  \begin{array}{l}
\frac{\partial r}{\partial l} = \frac{-2r\cos\theta}{R_E\sin^3\theta} \left( \frac{r^2\sin^3\theta}{R_E(1+3\cos^2\theta)}\right) = \frac{-2r^3\cos\theta}{R_E(1+3\cos^2\theta)} \\

\frac{\partial r}{\partial q} =  \frac{R^2_E\sin\theta}{r^2} \left( \frac{r^2\sin^3\theta}{R_E(1+3\cos^2\theta)}\right) = \frac{R_E\sin^4\theta}{(1+3\cos^2\theta)} \\

\frac{\partial r}{\partial \xi} = 0 
    \end{array}
     \right.
\label{sc_parar}
\end{equation}

Best Answer

The following uses the idea from Sub-numbering equations within array and the cases package:

enter image description here

\documentclass{article}
\usepackage{cases}% http://ctan.org/pkg/cases
\renewcommand{\theequation}{\thesection.\arabic{equation}}
\renewcommand{\thesubequation}{\themainequation\alph{equation}}
\newcommand{\pderiv}[2]{\frac{\partial #1}{\partial #2}}
\begin{document}

\section{Some stuff}

\setcounter{equation}{27}
\begin{equation}
  \left\{\begin{array}{@{}r@{}l@{}}
    \displaystyle\pderiv{x}{\zeta} & {}= \mbox{(\ref{A}) and (\ref{X})} \\
    \displaystyle\pderiv{y}{\zeta} & {}= \mbox{(\ref{B}) and (\ref{Y})} \\
    \displaystyle\pderiv{z}{\zeta} & {}= \mbox{(\ref{C}) and (\ref{Z})}
  \end{array}\right.
\end{equation}

\setcounter{equation}{33}

\begin{subnumcases}{}
  \pderiv{r}{l}     = A \label{A} \\
  \pderiv{r}{q}     = B \label{B} \\
  \pderiv{r}{\zeta} = C \label{C}
\end{subnumcases}

And then some text.
\begin{subnumcases}{}
  \pderiv{\theta}{l}     = X \label{X} \\
  \pderiv{\theta}{q}     = Y \label{Y} \\
  \pderiv{\theta}{\zeta} = Z \label{Z}
\end{subnumcases}

\end{document}

Here is a duplicate implementation using empheq:

enter image description here

\documentclass{article}
\usepackage{empheq}% http://ctan.org/pkg/empheq (autoloads amsmath)
\renewcommand{\theequation}{\thesection.\arabic{equation}}
\newcommand{\pderiv}[2]{\frac{\partial #1}{\partial #2}}
\begin{document}

\section{Some stuff}

\setcounter{equation}{27}
\begin{equation}
  \left\{\begin{array}{@{}r@{}l@{}}
    \displaystyle\pderiv{x}{\zeta} & {}= \mbox{(\ref{A}) and (\ref{X})} \\
    \displaystyle\pderiv{y}{\zeta} & {}= \mbox{(\ref{B}) and (\ref{Y})} \\
    \displaystyle\pderiv{z}{\zeta} & {}= \mbox{(\ref{C}) and (\ref{Z})}
  \end{array}\right.
\end{equation}

\setcounter{equation}{33}

\begin{subequations}
\begin{empheq}[left=\empheqlbrace]{align}
  \pderiv{r}{l}     &= A \label{A} \\
  \pderiv{r}{q}     &= B \label{B} \\
  \pderiv{r}{\zeta} &= C \label{C}
\end{empheq}
\end{subequations}

And then some text.
\begin{subequations}
\begin{empheq}[left=\empheqlbrace]{align}
  \pderiv{\theta}{l}     &= X \label{X} \\
  \pderiv{\theta}{q}     &= Y \label{Y} \\
  \pderiv{\theta}{\zeta} &= Z \label{Z}
\end{empheq}
\end{subequations}

\end{document}
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