I have
bla $ H $ bla bla $ 2 \times 2 $ . blabla:
\begin{equation}
H = \theta \textbf{I} + \textbf{ \tau } \cdot \textbf{a},
\end{equation}
Why does it give me the following error:
! Missing $ inserted.
<inserted text>
$
l.206 H = \theta \textbf{I} + \textbf{ \tau }
\cdot \textbf{a},
Can anyone help?
\textbf{\tau}is wrong. It should be\bm{\tau}(and you need\usepackage{bm}. Also\textbf{I}and\textbf{a}should be\mathbf{I}and\mathbf{a}. – egreg Jun 08 '15 at 16:26