How to left align an equation, with a brace as left delimiter?

Question

This is my code:

\begin{document}
$$
\sigma(s,i) = \left\{
\begin{array}{ll}
    \tau_{si} & \mbox{si } \{s,i\} \in E \\
    \infty & \mbox{sinon.}
\end{array}
\right.
$$
\end{document}

I have the equation in the center. How can I modify the code to have the equation to the left ?

Just this one equation or every displayed equation? If the latter, use \usepackage[fleqn]{amsmath} and please, don't use $$...$$ but \[...\]. — Gonzalo Medina, May 24 '13 at 18:45
... and please, don't use \left\{ and array and \right. but \begin{cases} your stuff \end{cases} — jmc, May 24 '13 at 18:48
@researcher, you might consider using other tags. Your code does not have much to do with {latex3}, but more with {equations}. — jmc, May 24 '13 at 18:51
@TVerron Thanks for updating the question title; it's a more appropriate one now. — Alan Munn, May 24 '13 at 19:29

score 3 · Accepted Answer · edited Jul 15 '17 at 06:45

3

You can use the fleqn option for amsmath:

\documentclass{article}
\usepackage[fleqn]{amsmath}

\begin{document}

\[
\sigma(s,i) = 
\begin{cases}
    \tau_{si},\quad &\text{si } \{s,i\} \in E \\
    \infty, & \text{sinon.}
\end{cases}
\]

\end{document}

The \quad command is optional (though recommended by some authors) and introduces some horizontal space between the expressions and the conditions.

Notice also the use of \[...\] instead of the $$....$$ TeX syntax and of the cases environment and the \text command (also from amsmath).

edited Jul 15 '17 at 06:45

ShreevatsaR

45,428
10
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answered May 24 '13 at 18:49

Gonzalo Medina

505,128

I think that the \quad should not be there, right? – jmc May 24 '13 at 18:53
1

@jmc why not? It's a common recommendation to leave a \quad space between the expression and the condition in a definition by cases. – Gonzalo Medina May 24 '13 at 18:53
I do not know why one would do that. Please enlighten me... – jmc May 24 '13 at 18:55
The necessity of such a \quad makes me think that cases was not designed well. I do not yet have a reason to think that. My experience so far has not caused me to think that such a \quad is needed. – jmc May 24 '13 at 19:01
@jmc in the end, it's just a matter of taste. Leaving a \quad space might increase readability in some cases. In any case, I'll add a note about the \quad being optional. – Gonzalo Medina May 24 '13 at 19:01
1

@jmc Take a look at http://tex.stackexchange.com/questions/25810/when-one-should-use-spacing-line-quad-or – May 24 '13 at 19:02
1

@Papiro I was just trying to find that answer, but you were faster. Thanks! – Gonzalo Medina May 24 '13 at 19:03

How to left align an equation, with a brace as left delimiter?

1 Answers1