\textbf in math expression returns error

Question

When I try to use the expression

$\textbf{a \cdot (a \times b)}$

It returns

! Missing $ inserted.<inserted text>$ Identiteten $ \textbf{a \cdot (a \times b)}

and lots of warnings like this:

! Extra }, or forgotten $.\textdef@ ...th {#1}\let \f@size 
#2\selectfont #3}}        Identiteten $ \textbf{a \cdot (a \times b)}

What is wrong with my code?

Answer 1

You're using the wrong markup: surround each variable by \mathbf:

$\mathbf{a} \cdot (\mathbf{a} \times \mathbf{b})$

Better yet, define a macro for your vectors:

\documentclass{article}
\usepackage{amsmath}

\newcommand{\vect}[1]{\mathbf{#1}}

\begin{document}

$\vect{a} \cdot (\vect{a} \times \vect{b})$

\end{document}

The result will be the same, but you have semantic markup, which is better and you'll know why when you'll need to modify how vectors are printed or to extend the definition for covering also Greek letters.

Answer 2

\textbf{} is meant for text mode only, i.e. non-math characters and symbols. To use a bold font (if really needed) \mathbf{} is to be used, for bold symbols like the \times \bm{} from bm package can be applied or the \boldmath command.

\documentclass{article}
\usepackage{bm}  % bold math 
\begin{document}
\begin{itemize}
\item Normal weight

$a \cdot (a \times b)$

\item Bold

$\mathbf{a \cdot (a \times b)}$

\item Bold symbols

$\mathbf{a \cdot (a \bm{\times} b)}$

\end{itemize}
\end{document}

If a and b are vectors, the dot product here is of course zero ;-)