LaTeX.org

Hi, guys. I am using Tex Live shipped with Ubuntu, and I have the following problem:
$G(p,C) = \left\{
\begin{array}{rcl}
]a,b[ & \mbox{if} & l_{-} \neq \phi \neq l_{+} \\
[p,b) & \mbox{if} & l_{-} = \phi \neq l_{+} \\
(a,p) & \mbox{if} & l_{-} \neq \phi = l_{+} \\
\{p\} & \mbox{if} & l ...