# Main label in a system of equations

I'm trying to create a system of equations with sub-numbering using the \begin{subnumcases} environment. Everything works perfectly but I would need to place a label somewhere in order to ref the system itself using the main number assigned to it, without having to ref one equation of it.

This is my code:

\documentclass{book}
\usepackage{cases}
\begin{document}
\begin{subnumcases}{}\label{eq:system}
\frac{d\psi}{d\zeta} = \frac{m_1}{y_1\prime}(x_1-\beta y_1\prime) \label{eq:dpsidz}
\\*
\frac{dx_i}{d\zeta} = \frac{m_i}{\psi y_i\prime}(x_i-y_i\prime)(x_i-\beta y_i\prime) \qquad \forall i=1,...,C-1 \label{eq:dxdz}
\\*
0 = y_1\prime (x_j-\beta y_j\prime)-\alpha_{1j}y_j\prime(x_1-\beta y_1\prime) \qquad \forall j=2,...,C \label{eq:algeb1}
\\*
0 = \sum_{i=1}^{C}y_i\prime -1 \label{eq:algeb2}
\\*
\psi|_{\zeta=0} = 1
\\*
x_i|_{\zeta=0} = x_i^F \qquad \forall i=1,...,C
\end{subnumcases}\\
The system illustrated in \ref{eq:system} is a \textsc{dae} system with a differential index equal to 1.
\end{document}


As you can see, I've tried to place the label{eq:system} inside the environment, before the first equation, but when I ref it in text it appears the name of the first equation. If the system equations are labelled 1.1a, 1.1b etc., I would like to ref the system as 1.1. I hope I was clear in explaining my issue. Thank you

You can use empheq together with amsmath and its subequations environment.

\documentclass{book}
\usepackage{amsmath,empheq}
\usepackage{hyperref}

\begin{document}

\begin{subequations}\label{eq:system}
\begin{empheq}[left=\empheqlbrace]{align}
&\frac{d\psi}{d\zeta} = \frac{m_1}{y_1'}(x_1-\beta y_1') \label{eq:dpsidz}
\\
&\frac{dx_i}{d\zeta} = \frac{m_i}{\psi y_i'}(x_i-y_i')(x_i-\beta y_i')
\\
&0 = y_1' (x_j-\beta y_j')-\alpha_{1j}y_j'(x_1-\beta y_1')
\\
&0 = \sum_{i=1}^{C}y_i' -1 \label{eq:algeb2}
\\
&\psi|_{\zeta=0} = 1
\\
&x_i|_{\zeta=0} = x_i^F \qquad \forall i=1,\dots,C
\end{empheq}
\end{subequations}
The system illustrated in \ref{eq:system} is a \textsc{dae} system
with a differential index equal to~$1$.

\end{document}


With hyperref the anchor will be immediately above the system. A couple of stylistic notes:

1. instead of ... you should input \dots;
2. instead of \prime you should use ^{\prime} or the easier ' (apostrophe) as in the code above.
• thank you, that solved the problem!! and thanks for the other tips as well! – Diego Sep 21 '18 at 10:02

Try putting \label{eq:system} inside the empty bracket next to \begin{subnumcases} as shown below. then you can refer it properly.

\begin{subnumcases}{\label{eq:system}}

• Thanks!! Now it works almost fine. The referred name is right, the only problem is that when I click on the referred eq. (i use hyperref) the position it shows me is not the system's position, but higher in the page. To be clear: the system is labelled 3.13 and when i click on it it sends me to eq. 3.12 which is before the system of course. – Diego Sep 20 '18 at 10:56
• That might be because you have really long equations, you could try using breaqn, as suggested in this post tex.stackexchange.com/questions/133773/… , also accept the answer to know others the problem is solved, thank you – Komal Pathade Sep 20 '18 at 11:12