Why am I getting a "$ inserted" error message?
The standalone
class, without the varwidth
or preview
option, just typesets the contents of the document
environment in a single horizontal box.
If you try
\mbox{\[a_1\]}
you incur in exactly the same error, because \[
issues $$
which, in restricted horizontal mode, just typesets an empty math formula and so _
is found outside of math mode.
If you want to use standalone
to get a printout of a single math formula in display style do
\documentclass[border=2pt]{standalone}
\begin{document}
$\displaystyle
B'(1)_n = B'(0)_{n+1}
$
\end{document}
Note that the preview
option will consider the formula to occupy a whole line. Compare the result with the suggested method
and the output of
\documentclass[border=2pt,preview]{standalone}
\begin{document}
\[
B'(1)_n = B'(0)_{n+1}
\]
\end{document}
Use the preview option. For displayed math one should use the preview package. It makes no sense to crop a displayed math expression. This is the reason why standalone doesn't support display math environments with the default crop option
\documentclass[border=2pt,preview]{standalone}