Highlight equations in a typographically correct manner

Question

I am presenting a model composed of a dozens of equations, some of them requiring a somewhat lengthy derivation.

The equation that continue the actual model appears progressively along the derivation in a sequence that I hope is logical. I would like to highlight those equations that constitute the actual model so that on coming back to the text the reader could skim the derivation and find the working equations.

I have tried several options but I am not really satisfied with them.

1. Framing the equation: the amsmath package provides the boxed command that allows to frame equation. While it indeed serves its purpose, it also degrades the overall look of the document. This have been debated here.

2. Margin sign -- using Koma's \marginline command could do. Except that my margin are already occupied by some other marks and cluttered margins do not look good either.

3. Restating -- restating all the important equation might be awkward to, resulting in a full page of equations (or even a few).
4. Appendix -- a variant of the previous that basically hide the awkwardness in the appendices.
5. Margin bar -- an idea I derived from this question would be to add a bar on the side of the important equations using an altered version of the leftbar environment (see example below).
6. Coloured background : as suggested by David Carlisle, a light gray background is not too bad (see example below).

Right now I think I am preferring option 3. or 5. but I was wondering if you would have something better to propose? Maybe some highlighting less drastic than framing, like bold equation number (I don't know how to achieve this)? What are accepted practices in publications?

Example to play with and see tested solutions :

    \documentclass{article}
    \usepackage{lipsum}
    \usepackage{amsmath}

    % Coloured background solution
    \usepackage{xcolor}
    \usepackage{mdframed}
    \newmdenv[backgroundcolor=black!15!white,
      linewidth=0pt,
      innerleftmargin=0pt,
      innerrightmargin=0pt]{graybck}


    % Margin bar solution
    \usepackage{framed}
    \newlength{\leftbarwidth}
    \setlength{\leftbarwidth}{1pt}
    \newlength{\leftbarsep}
    \setlength{\leftbarsep}{10pt}
    \renewenvironment{leftbar}{%
      \def\FrameCommand{{\vrule width \leftbarwidth\relax\hspace {\leftbarsep}}}%
      \MakeFramed {\advance \hsize -\width \FrameRestore }%
    }{%
      \endMakeFramed
    }



    \newcommand{\highlighteq}[1]{#1}
    \newenvironment{highlighteqenv}{}{}

    \begin{document}
    He said : ` you must defeat Sheng Long to stand a chance' which more or less amounted to
    \begin{highlighteqenv}
      \begin{equation}
        \highlighteq{
          \frac{\partial \langle \alpha K \rangle}{\partial t} + \frac{\partial \lambda}{\partial z} =0
                    },
      \end{equation}
    \end{highlighteqenv}

    \lipsum[1]
    \begin{equation}
      k_e =  42 \times \int x \ dt ,
    \end{equation}
    and
    \begin{equation}
       k_z =  42 \times \int u \ dt.
    \end{equation}



    To this one would object that :
    `\lipsum[2]'

    \begin{equation}
      \frac{\partial \langle \alpha K \rangle}{\partial \lambda} = 42 .
    \end{equation}

    Xyzzy !

    \begin{highlighteqenv}
      \begin{equation}
        \highlighteq{H_{appens} \rightarrow 0} .
      \end{equation}
    \end{highlighteqenv}


    \lipsum[3]
    \begin{equation}
      \sum [ \bar{\alpha} K' \mu ] = 42.1 ,
    \end{equation}
    and
    \begin{highlighteqenv}
      \begin{equation}
        \highlighteq{y^2 + ax + b = 39.9} .
      \end{equation}
    \end{highlighteqenv}

    \end{document}

Answer 1

I have been asking myself the same question for a long time, and finally came up with the idea of inserting a star symbol inside the main equation numbers, as explained in this thread. I have not yet had the opportunity to submit a paper using this technique, but guess that typical journals would accept it.