margin note left of an equation? [closed]

Question

How is it possible to place a margin note left of an equation? I'm using scrartcl from KOMAScript.

I set \reversemarginpar, then I tried with \marginnote from the package marginnote and also with Latex's own \marginpar and with \marginline from KOMAScript:

\documentclass{scrartcl}

\usepackage{mathtools}
\usepackage{marginnote}

\reversemarginpar

\begin{document}
\marginnote{test} \begin{align*}
  a &= b \\
  c &= d
\end{align*} \marginnote{test2}
\end{document}

The first marginnote (test) is left of the line before the align and the test2 note is next to line after the align. A margin note in the align gives errors. I know I can adjust the vertical offset with marginnote, but that means I have to do it manually (what is not a very good solution IMHO).

How can I place a margin note next to an equation?

Answer 1

You can use \tag to use an equation label, but offset it into the margin:

\documentclass{scrartcl}

\usepackage{mathtools}
\usepackage{marginnote}
\newcommand\mathnote[1]{\tag*{\llap{\smash{\parbox[t]{\marginparwidth}{%
\raggedright#1}%
\setbox0\hbox{)}%
\kern\dimexpr\marginparsep+\displaywidth-\wd0\relax}}}}
\reversemarginpar

\begin{document}
\noindent X\dotfill X

\marginnote{test} \begin{align*}
  a &= b\mathnote{test3 . . . . . . . . } \\
  c &= d 
\end{align*} \marginnote{test2 . . . . . . . . . }
\end{document}