# [Tex/LaTex] Two double-line equations need double left brackets

arraysbracketsequations

I have four lines of equations. The first two and the second two need to be grouped by a left bracket, and all four need to be grouped by one overall left bracket.

I am using an array within an equation. I can't manage to add the two 'inner' left brackets. The one big left bracket over all four equations is not a problem, which is the following:

$$\left\{ \begin{array}{lcl} 1\\ 2\\ 3\\ 4 \end{array} \right.$$


And this seems to work just fine. But when I try the following

$$\left\{ \begin{array}{lcl} \left\{ \begin{array}{lcl} 1\\ 2 \end{array}{lcl} \right. \end{array} \begin{array}{lcl} \left\{ \begin{array}{lcl} 3\\ 4 \end{array} \right. \end{array} \right.$$


it doesn't work. I've tried some other changes, including changing to eqnarray, or changing the places of the brackets, but nothing seems to work. Now I am not sure how to solve this.

I think that some information about the actual expressions might be useful to decide which structure(s) would be best to use; in its present form (using arrays for the two subgroups), one could use a cases environment for the outer brace:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$\begin{cases} \left\{ \begin{array}{lcl} 1 \\ 2 \end{array} \right. \\ \left\{ \begin{array}{lcl} 3 \\ 4 \end{array} \right. \end{cases}$$

\end{document}


Here's another option, using a gather environment for the whole structure, and nested arrays:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{gather}
\left\{
\begin{array}{@{}l@{}}
\left\{
\begin{array}{lcl}
1 \\
2
\end{array}
\right. \\
\left\{
\begin{array}{lcl}
3 \\
4
\end{array}
\right.
\end{array}
\right.
\end{gather}

\end{document}


By the way, your code has some typos like \end{array{lcl}.