Wick contraction in a split equation

Question

I have a long expression over many lines for which I want to insert a few Wick contractions. To be clear each wick contraction will stay on a single line but the whole expression is many. I find that as soon as I try insert a wick contraction in a split environment with more then one line I cannot get a wick contraction. I have tried simple wick and simpler-wick packages. I would like to get something like what I have below:

\documentclass{article}
\usepackage{simpler-wick}
\begin{document}    
\begin{equation}
\begin{split}
 \wick{AB & =CDEF \\ & = \c G HI \c J}
\end{split}
\end{equation}
\end{document}

Is this possible? Thanks

Answer 1

Welcome to TeX.SE!

NOTE: Update your TeX distribution before you run the code below. If you are using TeXLive, the following should work. If you are using another TeX distribution, you may have to download the tikzmark library from here.

In the example you post, simpler-wick does work because you only do a contraction within one block. That is, you only need to move \wick to the block in which the both \c of your contractions are.

\documentclass{article}
\usepackage{simpler-wick}
\usetikzlibrary{tikzmark}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{split}
 \tikzmarknode{A}{A}B & =C\tikzmarknode{D}{D}EF \\ & = \wick{\c1G HI \c1J}
\end{split}
\begin{tikzpicture}[overlay,remember picture]
\draw ([yshift=2pt]A.north) -- ++(0,3pt) -| ([yshift=2pt]D.north);
\end{tikzpicture}
\end{equation}
\end{document}

It will always work as long as the contraction does not span over a & or a line break. Most of the contractions in the physics literature will hence work as they typically do not go over equality signs nor over two lines, but of course there may be situations where this is different. In this case, you may build your own. You are very lucky that the new version of the tikzmark library has just been uploaded to CTAN and is already in the TeXLive distribution. So, in order for this to work, you need to update your TeX distribution. Using \tikzmarknode you can draw the contraction in the fist line, which runs over a & sign.

Answer 2

For fun, it's very easy to do with only pst-node and auto-pst-pdf, without disturbing the equation layout:

\documentclass{article}
\usepackage{amsmath}
\usepackage{pst-node, auto-pst-pdf}

\begin{document}

\begin{postscript}
  \begin{equation}
    \begin{split}
     \Rnode{A}{A}B & =C\Rnode{D}{D}EF \\
     & = \psDefBoxNodes{GJ}{GHIJ}
    \end{split}
\end{equation}
\psset{linewidth=0.4pt, angle=90, arm=0.7ex, nodesep =1pt}
\ncbar{A}{D}
\ncbar[offsetA=-0.4em, offsetB=-0.15em]{GJ:tl}{GJ:tr}
\end{postscript}

\end{document}