subequations and array in braces

Question

I need something like this:

\left\{
first line  (1a)
second line (1b)
third line  (1c)
\right.

all lines should be aligned to left

Answer 1

(Update according to egreg's commend. Thanks!)

The first example of empheq given by Bernard is invalid if omitting the overload option when loading empheq package. For the latex file provided by Bernard, it has no problems at all. However, when I used \usepackage[overload]{empheq} in IEEEtrans with a lot other packages loaded, it will freeze the pdflatex on my computer, I cannot see why, so I loaded it without overload option. Here is the version when loading empheq without overloadoption (\empheqlbrace is the big left parenthesis. \, is just used to add extra space; the default is too tight.):

\begin{subequations}
    \begin{empheq}[left={K(A)=\empheqlbrace\,}]{align}
      & (I \otimes e^A) \psi\left(A^T \oplus (-A) \right) \\
      & (e^{A^T/2} \otimes e^{A/2}) \operatorname{sinch}\left(\frac{1}{2}[A^T \oplus (-A)]\right) \\
      & \frac{1}{2}(e^{A^T} \oplus e^A) \tau\left(\frac{1}{2}[e^T \oplus (-A)]\right),
    \end{empheq}
\end{subequations}

whose output looks like:

---

For a question on citations, it works out of box, e.g. see the following codes:

\documentclass[a4paper,10pt]{article}

\usepackage{amsmath}
\usepackage{empheq}

\begin{document}
We refer to the subequation (\ref{eq:2}) and the whole equation (\ref{eq:all}).
\begin{subequations}
  \label{eq:all}
    \begin{empheq}[left={K(A)=\empheqlbrace\,}]{align}
      & (I \otimes e^A) \psi\left(A^T \oplus (-A) \right)
        \label{eq:1} \\
      & (e^{A^T/2} \otimes e^{A/2}) \operatorname{sinch}\left(\frac{1}{2}[A^T
        \oplus (-A)]\right)
        \label{eq:2}\\
      & \frac{1}{2}(e^{A^T} \oplus e^A) \tau\left(\frac{1}{2}[e^T \oplus
        (-A)]\right),
        \label{eq:3}
    \end{empheq}
\end{subequations}
\end{document}

which gives

Answer 2

Two solutions: one with the empheq package and an align environment, another with the subnumcases environment from the cases package. I had to adjust spacing for the brace in both, as I found the default was not so good (too small in the first case, too large in the second):

    \documentclass{article}

    \usepackage[utf8]{inputenc}
    \usepackage[overload]{empheq}
    \usepackage{cases} 

    \begin{document}

    \begin{subequations}
    \begin{align}[left = \empheqlbrace\,]
      & a = b \\
       & c = d
    \end{align}
    \end{subequations}

    \begin{subnumcases}{}
    \!\! a = b \\
    \!\! c = d
    \end{subnumcases}

    \end{document}