Error using equation*

Question

I get an error in the following:

\documentclass[12pt,a4paper,english]{article} 
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\left\{\begin{matrix}
u_t+vu_x=0 & x \in \mathbb{R}\\ 
g(x)=u(x,0)=\textrm{exp}(-10x^2) & x \in \mathbb{R}
\end{matrix}\right
\end{equation*}
\end{document}

I get the error

 Blockquote Missing delimiter (. inserted).
<to be read again> 
                   $
l.12 \end{equation*}                  
I was expecting to see something like `(' or `\{' or
`\}' here. If you typed, e.g., `{' instead of `\{', you
should probably delete the `{' by typing `1' now, so that
braces don't get unbalanced. Otherwise just proceed.
Acceptable delimiters are characters whose \delcode is
nonnegative, or you can use `\delimiter <delimiter code>'.

What's the problem? I don't see why it complains for the \end{equation*}.

Answer 1

The problem is solved using \right. (where . denotes the “empty delimiter”). But you should use cases.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{equation*}
\left\{\begin{matrix}
u_t+vu_x=0 & x \in \mathbb{R}\\
g(x)=u(x,0)=\textrm{exp}(-10x^2) & x \in \mathbb{R}
\end{matrix}\right.
\end{equation*}

\begin{equation*}
\begin{cases}
u_t+vu_x=0               & x \in \mathbb{R}\\
g(x)=u(x,0)=\exp(-10x^2) & x \in \mathbb{R}
\end{cases}
\end{equation*}

\end{document}

Also it's not \textrm{exp}, but \exp (which is predefined). See What's the difference between \mathrm and \operatorname? for more information why.