我如何进行 Wick 收缩？

Question

我可以用 LateX 制作这个吗？我想用这些线做组号。有人能帮我吗？

Answer 1

这非常简单pstricks– 更准确地说是pst-node：

\documentclass{article}
\usepackage{mathtools}
\usepackage{pst-node} 
\usepackage{auto-pst-pdf} 

\begin{document}

\[ \rnode{X1}{x^2}y + \rnode{XY}{3xy} + \rnode{X2}{2x} + \rnode{C}{6} \]%
\ncbar[nodesep=0.5ex, arm=1.5ex, angle=-90]{X1}{X2}
\ncbar[nodesepB=0.5ex, angle=-90]{XY}{C}

\end{document} 

Answer 2

和tikzmark：

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\begin{document}
    \[
\tikzmarknode{A}{x}^2 y
    + 3\tikzmarknode{B}{x}y + 2\tikzmarknode{C}{x} + \tikzmarknode{D}{6}
%
\begin{tikzpicture}[overlay, remember picture,
                    every path/.style = {shorten <=1pt,shorten >=1pt}
                    ]
\draw   (A.south) -- ++ (0,-0.2) -| (C);
\draw   (B.south) -- ++ (0,-0.4) -| (D);
\end{tikzpicture}
    \]
\vspace{4mm} % space for tikzpicture
text text text text text text text text text text text text
\end{document}

经过两次编译你将得到：

Answer 3

拥有simpler-wick和以下补丁你可以做

\documentclass[fleqn]{article}
\usepackage[sep=3pt,offset=0.5ex]{simpler-wick}
\newif\ifWickBelow
\WickBelowfalse
\pgfkeys{
  /simplerwick/below/.code={\WickBelowtrue},
}

\makeatletter
\def\swick@end#1#2{
  \swick@setfalse@#1
  \tikzexternaldisable
  \begin{tikzpicture}[remember picture, baseline=(swick-close#1.base)]
    \node[use as bounding box, inner sep=0pt, outer sep=0pt] (swick-close#1) {$\displaystyle #2$};
  \end{tikzpicture}
  \tikz[remember picture, overlay]
{
\ifWickBelow
    \draw ($(swick-open#1.south) + (0, -3pt)$) 
          -- ($(swick-open#1.base) + (0, -\swick@offset) + #1*(0, -\swick@sep)$) 
          -- ($(swick-close#1.base) + (0, -\swick@offset) + #1*(0, -\swick@sep)$) 
          -- ($(swick-close#1.south) + (0, -3pt)$);
\else
    \draw ($(swick-open#1.north) + (0, 3pt)$) 
          -- ($(swick-open#1.base) + (0, \swick@offset) + #1*(0, \swick@sep)$) 
          -- ($(swick-close#1.base) + (0, \swick@offset) + #1*(0, \swick@sep)$) 
          -- ($(swick-close#1.north) + (0, 3pt)$);
\fi}
\tikzexternalenable}
\makeatother

\begin{document}
\[\wick[below]{x\c1{\widehat{~}}2 y+3\c2xy+2\c1 x+\c26}\]
\end{document}

有几种替代方法来做 Wick 收缩，例如这里。