boldmath-mode
This code
\begin{equation}
\label{eq:test}
| \mathbf{v} |
\end{equation}
gives me the error:
ERROR: LaTeX Error: \mathbf allowed only in math mode.
--- TeX said ---
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.430 | \mathbf
{v} |
My question is simply why? Is not equation math mode??
Before you load the offending package, put
\let\Horig\H
and then you can say Erd\Horig{o}s
You're missing a backslash character, \, between \left and {.
However, rather than just adding the missing backslash character, you may want to rewrite the entire expression to make use of the cases environment to obtain an overall better format.
\documentclass{scrbook}
\usepackage{amsmath}
\begin{document}
Original form, with backslash inserted after second \texttt{\textbackslash left}:
\[
\left| x \right|=
\left\{
\begin{matrix} x, if x\geq 0\\ -x, if x<0 \end{matrix}
\right.
\]
\bigskip
Modified form, using \texttt{cases} environment:
\[
\left | x \right | =
\begin{cases}
\hfill x & \text{if $x\geq 0$}\\
-x & \text{if $x<0$}
\end{cases}
\]
\end{document}
Best Answer
You already know that many packages redefine
|. Thus I usually recommend usingamsmathand its commands\lvertand\rvertfor left and right pipe delimiters. Besides protection against redefinition of|, this also makes better spacing when|is indeed a delimiter.