How to include a system of equations in a subequation environment?

Question

I am wondering how to include a system of equations in a subequations environment.

I currently have the following document as an example :

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{subequations} 
\begin{alignat}{2}
& &&\quad a =1 \\
& &&\quad b =2 \\
& &&\quad c
\end{alignat}
\end{subequations}
\end{document}

Here is the display :

Now, what I would like to have is the first two equation groupped into a system of equations (with a brace) and only one label to this system. Something looking like this :

Any suggestions ?

Answer 1

Depending on your requirements for alignment, the following should work:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{subequations} 
  \begin{align}
    \forall l \in L, & \quad a = 1 \\
    \forall l \in L, & \quad b = 2 \\
    \forall l \in L, & \quad c
  \end{align}
\end{subequations}

\begin{subequations} 
  \begin{align}
    &{\left.\kern-\nulldelimiterspace
    \renewcommand{\arraystretch}{1.5}
    \setlength{\arraycolsep}{0pt}
    \begin{array}{l}
      \forall l \in L, \quad a = 1 \\
      \forall l \in L, \quad b = 2
    \end{array}
    \right\}} \\
    &\forall l \in L, \quad c 
  \end{align}
\end{subequations}

\end{document}