# Highlight equations in a typographically correct manner

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}
$$\highlighteq{ \frac{\partial \langle \alpha K \rangle}{\partial t} + \frac{\partial \lambda}{\partial z} =0 },$$
\end{highlighteqenv}

\lipsum[1]
$$k_e = 42 \times \int x \ dt ,$$
and
$$k_z = 42 \times \int u \ dt.$$

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

$$\frac{\partial \langle \alpha K \rangle}{\partial \lambda} = 42 .$$

Xyzzy !

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

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

\end{document}

-
Is (foreground or background) colour an option? – David Carlisle Sep 15 '12 at 16:15
In my opinion, background color is equivalent to framing but even more 'bold' and might causes printig issues (except maybe a very light gray? It could be tried). Foreground could be a good idea but most people print in black and white. – Alfred M. Sep 15 '12 at 16:39
Yes that's why I asked first, colour works well for on screen delivery or for high quality printing, but if it's going to be printed on the office b&w laserprinter, not so much:-) – David Carlisle Sep 15 '12 at 16:57
I suspect that you are going to have to use a variety of the techniques. I normally prefer an appendix for an lengthy derivations that would get in the way of the flow of the text. With the boxed versions, there are things to be aware of when you want to box just one equation in an align environment. See Highlight an equation within an align environment for example. – Peter Grill Sep 15 '12 at 18:49
One way would be to give special tags (MFCQ, KKT, etc.) to the relevant formulas using the \tag command. – bzklrm Sep 16 '12 at 0:25

I would even say that his idea is better. For example, we often put axioms of some algebraic structure as (as1)--(as5) etc. And the same can be done with any equation. With stars, you'll easily run out of them ;) And for your last point, yes journals have no problem with named tags, as long as you don't change all of them ;) – yo' Sep 27 '12 at 7:57