5

I have a big equation that I want to split with empheq :

\begin{empheq}[right={\empheqrbrace =  - \oint_S \frac{\overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS - \oint_S \frac{ \overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS }]{align*}
  \text{si $V \in V_p$,}& 0  \\
  \text{si $V \notin V_p$,}& \vec{v}_p
\end{empheq}

I have tried to break my equation with \\ and \newline but it doesn't work ... ?

  • ok, it works. Mico response is the simpliest for me. thank you everyone – Kafka Jan 6 '17 at 16:37
  • Just to explain why this does not work: what you essentially have here is similar to \begin{equation*} \begin{aligned} TEXT\end{aligned} \} = RIGHT \end{equation*}, so as the others have pointed out, you probably want to do this differently. – daleif Jan 6 '17 at 16:38
3

I would use an align* environment and a single line break directive. I wouldn't autosize the parentheses, and I'd use \bar instead of \overline. For better legibility, if you're using Computer Modern as the document font, consider inserting \, (thinspace) if \vec{r} or \vec{\xi} is followed by a closing parenthesis.

enter image description here

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\left.\begin{array}{ll}
\text{si $V \in V_p$,    } & 0  \\[1ex]
\text{si $V \notin V_p$, } & \vec{v}_p
\end{array}\right\} 
&= - \oint_S \frac{\bar{\bar{G}}(\vec{r}-\vec{\xi}\,)}{8\pi \mu}\bigl(\bar{\bar{\sigma}}(\vec{r}\,)\vec{n}\bigr)dS 
+ \oint_S \vec{v}(\vec{r}\,)\biggl(\frac{\bar{\bar{\bar{\Sigma}}}(\vec{r}-\vec{\xi}\,)}{8\pi \mu}\vec{n}\biggr)dS \\
&\quad - \oint_S \frac{ \bar{\bar{G}}(\vec{r}-\vec{\xi}\,)}{8\pi \mu}\bigl(\bar{\bar{\sigma}}(\vec{r}\,)\vec{n}\bigr)dS  
+ \oint_S \vec{v}(\vec{r}\,)\biggl(\frac{\bar{\bar{\bar{\Sigma}}}(\vec{r}-\vec{\xi}\,)}{8\pi \mu}\vec{n}\biggr)dS 
\end{align*}
\end{document}
4

Not with empheq, but with more standard tools:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\ov}[1]{\overline{#1}}
\newcommand{\oov}[1]{\overline{\overline{#1}}}
\newcommand{\ooov}[1]{\overline{\overline{\overline{#1}}}}

\begin{document}

\begin{equation*}
\begin{rcases}
  \text{si $V \in V_p$,}    & 0  \\
  \text{si $V \notin V_p$,} & \vec{v}_p
\end{rcases}
=\begin{aligned}[t]
& - \oint_S \frac{\oov{G}(\vec{r} - \vec{\xi})}{8\pi \mu}
    (\oov{\sigma}(\vec{r})\vec{n})\,dS
  + \oint_S \vec{v}(\vec{r})\Biggl(\frac{\ooov{\Sigma}(\vec{r} - \vec{\xi})}{8\pi \mu}
    \vec{n}\Biggr)\,dS
\\
& - \oint_S \frac{ \oov{G}(\vec{r} - \vec{\xi})}{8\pi \mu}
    (\oov{\sigma}(\vec{r})\vec{n})\,dS  
  + \oint_S \vec{v}(\vec{r})\Biggl(\frac{\ooov{\Sigma}(\vec{r} -\vec{\xi})}{8\pi \mu}
    \vec{n}\Biggr)\,dS
\end{aligned}
\end{equation*}

\end{document}

enter image description here

With some refinements, basically adding \, when a vector is immediately followed by a parenthesis:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\ov}[1]{\overline{#1}}
\newcommand{\oov}[1]{\overline{\overline{#1}}}
\newcommand{\ooov}[1]{\overline{\overline{\overline{#1}}}}

\begin{document}

\begin{equation*}
\begin{rcases}
  \text{si $V \in V_p$,}    & 0  \\
  \text{si $V \notin V_p$,} & \vec{v}_p
\end{rcases}
=\begin{aligned}[t]
& - \oint_S \frac{\oov{G}(\vec{r} - \vec{\xi}\,)}{8\pi \mu}
    (\oov{\sigma}(\vec{r}\,)\vec{n}\,)\,dS
  + \oint_S \vec{v}(\vec{r})\Biggl(\,\frac{\ooov{\Sigma}(\vec{r} - \vec{\xi}\,)}{8\pi \mu}
    \vec{n}\Biggr)\,dS
\\
& - \oint_S \frac{ \oov{G}(\vec{r} - \vec{\xi}\,)}{8\pi \mu}
    (\oov{\sigma}(\vec{r}\,)\vec{n}\,)\,dS  
  + \oint_S \vec{v}(\vec{r}\,)\Biggl(\,\frac{\ooov{\Sigma}(\vec{r} -\vec{\xi}\,)}{8\pi \mu}
    \vec{n}\Biggr)\,dS
\end{aligned}
\end{equation*}

\end{document}

enter image description here

3

I don't know if this proposed alternative might suit, using stacks, rather than empheq?? Note I left the math in \textstyle as you originally had it.

\documentclass{scrartcl}

\usepackage{empheq,tabstackengine}

\begin{document}

Was:

\begin{empheq}[right={\empheqrbrace =  - \oint_S \frac{\overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS - \oint_S \frac{ \overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS }]{align*}
  \text{si $V \in V_p$,}& 0  \\
  \text{si $V \notin V_p$,}& \vec{v}_p
\end{empheq}

Proposed:

\[
\left.
\begin{aligned}
  \text{si $V \in V_p$,}& 0  \\
  \text{si $V \notin V_p$,}& \vec{v}_p
\end{aligned}
\right\}=
\ensurestackMath{\Shortunderstack
{- \oint_S \frac{\overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS 
\\ 
-\oint_S \frac{ \overline{\overline{G}}(\vec{r}-\vec{\xi})}{8\pi \mu}\left(\overline{\overline{\sigma}}(\vec{r})\vec{n}\right)dS  + \oint_S \vec{v}(\vec{r})\left(\frac{\overline{\overline{\overline{\Sigma}}}(\vec{r}-\vec{\xi})}{8\pi \mu}\vec{n}\right)dS }}
\]
\end{document}

enter image description here

If one wanted the equation in \displaystyle, then

  1. add \TABstackMathstyle{\displaystyle} to the preamble;

  2. change \ensurestackMath to \ensureTABstackMath; and

  3. change \Shortunderstack to \tabbedShortunderstack.

Here is the result:

enter image description here

2

A variant, always with rcases, using the \widebar command, borrowed from mathabx, and the bvector arrows from esvect, which look nicer than the basic vec, in my opinion:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{empheq}
\usepackage[f]{esvect} %
\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{
<5><6><7><8><9><10>
<10.95><12><14.4><17.28><20.74><24.88>
mathx10
}{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\DeclareMathSymbol{\bigboxvoid}{\mathop}{mathx}{"DC}
\DeclareMathAccent{\widebar}{0}{mathx}{"73}

\begin{document}

\begin{align*}
  \begin{rcases} \text{si } V ∈ V_p , & 0 \\
  \text{si }V ∉ V_p, & \vv{v}_p
  \end{rcases}
  = & - ∮_S \frac{\widebar{\widebar{G}}(\vv{r}-\vv{ξ})}{8πμ}\left(\widebar{\widebar{σ}}(\vv{r})\vv{n}\right)dS + ∮_S \vv{v}(\vv{r})\left(\frac{\widebar{\widebar{\widebar{Σ}}}(\vv{r}-\vv{ξ})}{8πμ}\vv{n}\right)dS \\
                                        & - ∮_S \frac{ \widebar{\widebar{G}}(\vv{r}-\vv{ξ})}{8πμ}\left(\widebar{\widebar{σ}}(\vv{r})\vv{n}\right)dS + ∮_S \vv{v}(\vv{r})\left(\frac{\widebar{\widebar{\widebar{Σ}}}(\vv{r}-\vv{ξ})}{8πμ}\vv{n}\right)dS
\end{align*}

\end{document} 

enter image description here

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