2

I'm trying to write

enter image description here

While combining the equation and cases ambient gives me something close

\begin{equation}\tag{PE}\label{PE}
\begin{cases}
-\Delta u&=f, \quad \text{em }\Omega \\
u&=0, \quad \text{em } \partial\Omega
\end{cases}
\end{equation}

enter image description here

it's not quite what I wanted (the positioning of the $u$ is super strange in the second line). I also tried using only the align ambient

\begin{align}\tag{PE}\label{PE}
-\Delta u &= f, \quad \Omega \\
u &=0, \quad \partial \Omega
\end{align}

enter image description here

but now I lose the bracket. How can I get that first image? Thanks in advance!

3 Answers 3

4

{cases} is a very useful environment but not quite flexible. Sometimes you just need to explicitly add a brace to the left of an {aligned} environment.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\left\{
\begin{aligned}
-\Delta u&=f, && \text{em $\Omega$} \\
        u&=0, && \text{em $\partial\Omega$}
\end{aligned}
\right.
\end{equation}

\end{document}

enter image description here

3

You can insert TeX primitive \hfill to the place where you want to have the stretchable space:

\begin{cases}
-\Delta u&=f, \quad \text{em }\Omega \\
\hfill  u&=0, \quad \text{em } \partial\Omega
\end{cases}
1

Here are two ways: one with an aligned environment nested in cases, another with the empheq environment from the eponymous package:

    \documentclass{article}

    \usepackage{empheq}

    \begin{document}

    \begin{equation}\tag{PE}\label{PE}
    \begin{cases}
    \begin{aligned}
    -\Delta u&=f, \quad \text{em }\Omega \\
    u&=0, \quad \text{em } \partial\Omega
    \end{aligned}
    \end{cases}
    \end{equation}

    \begin{empheq}[left=\empheqlbrace]{equation}\tag{PE}\label{PE}
    \begin{aligned}
    -\Delta u&=f, &\quad & \text{em }\Omega \\
    u&=0, & & \text{em } \partial\Omega
    \end{aligned}
    \end{empheq}

    \end{document} 

enter image description here

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