2

I am trying to imbricate empheq environments. The result (using array) I would like to obtain is :

enter image description here

I do not like array environment because it does not manage the spaces well, as you may see.

I therefore use empheq. And I obtain :

enter image description here

Do you know how to solve the problem ?

Regards

\documentclass[9pt]{extbook}
\usepackage[french]{keystroke}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage[table]{xcolor}

\usepackage{array,multicol}

\usepackage[T1]{fontenc}
\usepackage{lmodern} 
\usepackage[francais]{babel}

\usepackage{empheq}
\usepackage{varwidth}

\begin{document}

\[
\left\{\begin{array}{l}
\left\{\begin{array}{l}
a = b \times c \\
 d = e \times f
\end{array}\right. \\
\left\{\begin{array}{l}
a = b \times c \\
 d = e \times f
\end{array}\right.
\end{array} \right.
\]

\begin{empheq}[left=\empheqlbrace]{align*}
 { \begin{varwidth}{20cm}
\begin{empheq}[left=\empheqlbrace]{align*}
    a = b \times c \\
  d = e \times f
 \end{empheq}
 \end{varwidth}} \\
{ \begin{varwidth}{20cm}
\begin{empheq}[left=\empheqlbrace]{align*}
    a = b \times c \\
  d = e \times f
 \end{empheq}
 \end{varwidth}}
 \end{empheq}

\end{document}
3

Since you're not numbering the equations separately, there's no need for empheq:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
\begin{cases}
  \begin{cases}
    a = b \times c \\
    d = e \times f
  \end{cases}
\\
\\[-3\jot]
  \begin{cases}
    a = b \times c \\
    d = e \times f
  \end{cases}
\end{cases}
\]
\end{document}

The \\ \\[-\3jot] is just a trick to give a vertical separation.

enter image description here

2

It suffices to use the cases environment. As I didn't see why you used varwidth, I simplified your code:

\documentclass[9pt]{extbook}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}

\usepackage{array}
\usepackage{amsfonts,amssymb}
\usepackage{empheq}

\usepackage[francais]{babel}

\begin{document}

\begin{empheq}[left=\empheqlbrace]{align*}
\begin{cases}
    a = b \times c \\
  d = e \times f
 \end{cases}
\\
\begin{cases}
    a = b \times c \\
  d = e \times f
 \end{cases}
\end{empheq}

\end{document} 

enter image description here

Btw, empheq loads mathtools that in turn loads amsmath, so you don't have to load the latter.

1

Here, I use stacks for the equations, and show two different methods for the braces. While the scalerel approach gives nicer height matching of the braces, the \left\{ approach provides for standard brace shapes. Obviously, extra spaces can be added in, as needed. I removed all possible spaces to provide minimal interbrace spacing.

\documentclass{article}
\usepackage[usestackEOL]{stackengine}
\stackMath
\usepackage{scalerel}
\begin{document}
Scalerel approach

\scalerel[2ex]{\{}{%
\def\stackalignment{l}%
  \stackanchor{%
    \scalerel[1.7ex]{\{}{\Centerstack{a=b\times c\\d = e\times f}}%
  }{%
    \scalerel[1.7ex]{\{}{\Centerstack{a=b\times c\\d = e\times f}}%
  }%
}

left\{ approach:

\(
\left\{%
\def\stackalignment{l}%
  \stackanchor{%
    \left\{\Centerstack{a=b\times c\\d = e\times f}\right.%
  }{%
    \left\{\Centerstack{a=b\times c\\d = e\times f}\right.%
  }%
\right.
\)
\end{document}

enter image description here

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