从符号中心开始的重叠下括号/上括号

Question

tikz我正在尝试以最通用的方式 用来实现这种类似括号的结果。

天真地使用\overbracket{}和\underbracket{}来自mathtools手动对齐黑客，例如

\[
\,\,\overbracket{\!\!\nabla A + \nabla A\!\!}\,\,
\]

缺点就是不能重叠。

到目前为止，我找到的最佳解决方案是使用tikz如下所示的 MWE。 问题在于，tikzpictures（似乎）需要overlay选项才能使图片间坐标正常工作 — 然而，这也破坏了数学流程。（请注意下图中的对齐）。

这个例子可能会有帮助：化学方程式也类似。 非常感谢。

---

\documentclass{article}

\usepackage{amsmath,amssymb}
\usepackage{tikz}
    \usetikzlibrary{calc}

\begin{document}

\begin{align*}
    \mathcal{G} &= \nabla A + \nabla A
\\  \mathcal{G} &=
    \begin{tikzpicture}[overlay, remember picture, baseline={(current bounding box.south)}]
        \node[anchor=base, inner sep=0pt] (G1) at (0,0) {$\nabla$};
    \end{tikzpicture}
    A+{}\,
    \begin{tikzpicture}[overlay, remember picture, baseline={(current bounding box.south)}]
        \node[anchor=base, inner sep=0pt] (G2) at (0,0) {$\nabla$};
        \draw[blue] (G2) -- +(0, .2) -- ($(G1) + (0, 0.2)$) -- (G1);
    \end{tikzpicture}
    A
\\  \mathcal{G} &= A + A
\end{align*}

\end{document}

Answer 1

您也可以使用该simplewick包相当简单地完成此操作。

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{simplewick}
\begin{document}
\[
\acontraction[.5ex]{\nabla_\mu A_\nu \nabla^\mu A^\nu - \nabla_\mu A_\nu \nabla^\nu A^\mu = }{\nabla}{A}{\nabla}
\acontraction[.5ex]{\nabla_\mu A_\nu \nabla^\mu A^\nu - \nabla_\mu A_\nu \nabla^\nu A^\mu = \nabla A \nabla A -}{\nabla}{A\nabla}{A}
\bcontraction[.5ex]{\nabla_\mu A_\nu \nabla^\mu A^\nu - \nabla_\mu A_\nu \nabla^\nu A^\mu = \nabla}{A}{\nabla}{A}
\bcontraction[.5ex]{\nabla_\mu A_\nu \nabla^\mu A^\nu - \nabla_\mu A_\nu \nabla^\nu A^\mu = \nabla A \nabla A - \nabla}{A}{}{\nabla}
\nabla_\mu A_\nu \nabla^\mu A^\nu - \nabla_\mu A_\nu \nabla^\nu A^\mu = \nabla A \nabla A - \nabla A \nabla A
\]
\end{document}

Answer 2

您可以使用tikzmark。通常这种方法的问题在于，要使用 来绘制内容overlay，因此可能会经过等式的其他部分。这里计算了一些垂直填充并插入以避免这种情况。

\documentclass{article}

\usepackage{amsmath,amssymb}
\usepackage{tikz}
\usetikzlibrary{tikzmark,calc}
\newcommand{\Contract}[3][]{\begin{tikzpicture}[remember picture]
 \tikzset{contract/.cd,#1}
 \def\pv##1{\pgfkeysvalueof{/tikz/contract/##1}}%
 \begin{scope}[overlay]
 \ifdim\pv{vsep}>0pt
  \draw[style/.expanded=\pv{style}]
   ([yshift=\pv{gap}]#2.north) -- ++ (0,\pv{vsep}) -|  
   ([yshift=\pv{gap}]#3.north);
 \else
  \draw[style/.expanded=\pv{style}]
    ([yshift=-\pv{gap}]#2.south) -- ++ (0,\pv{vsep}) -|   
    ([yshift=-\pv{gap}]#3.south);
 \fi 
 \end{scope}
 \ifdim\pv{vsep}>0pt
  \path let \p1=($(#2.north)-(#2.base)$),
   \n1={\y1+\pv{gap}+\pv{vsep}-height("A")} 
   in node[baseline={(#2.base)}]{\vphantom{\raisebox{\n1}{.}}};
 \else
  \path let \p1=($(#2.base)-(#2.south)$),
  \n1={\y1-\pv{gap}+\pv{vsep}+2pt} 
   in node[baseline={(#2.base)}]{\vphantom{\raisebox{\n1}{.}}}; 
  \fi 
\end{tikzpicture}%
}
\tikzset{contract/.cd,vsep/.initial=3pt,gap/.initial=1pt,style/.initial={-,thick}}
\begin{document}

\begin{align*}
    \mathcal{G} &= \nabla A + \nabla A \\
    \mathcal{G} &= \tikzmarknode{n1}{\nabla} A
        +\tikzmarknode{n2}{\nabla} A    \Contract{n1}{n2} \\
  \mathcal{G} &= A + A
\end{align*}
\[  
  \nabla_\mu A_\nu \nabla^\mu A^\nu-
  \nabla_\mu A_\nu \nabla^\nu A^\mu
  =
  \tikzmarknode{n3}{\nabla}\tikzmarknode{A3}{A}
  \tikzmarknode{n4}{\nabla}\tikzmarknode{A4}{A}
  +
  \tikzmarknode{n5}{\nabla}\tikzmarknode{A5}{A}
  \tikzmarknode{n6}{\nabla}\tikzmarknode{A6}{A}
  \Contract{n3}{n4} \Contract[vsep=-3pt]{A3}{A4} 
   \Contract{n5}{A6} \Contract[vsep=-3pt]{n6}{A5}
\]

\[  
  \nabla_\mu A_\nu \nabla^\mu A^\nu-
  \nabla_\mu A_\nu \nabla^\nu A^\mu
  =
  \tikzmarknode{m3}{\nabla}\tikzmarknode{B3}{A}
  \tikzmarknode{m4}{\nabla}\tikzmarknode{B4}{A}
  +
  \tikzmarknode{m5}{\nabla}\tikzmarknode{B5}{A}
  \tikzmarknode{m6}{\nabla}\tikzmarknode{B6}{A}
  \Contract[style={blue,thick}]{m3}{m4} \Contract[vsep=-3pt,style={red,thick}]{B3}{B4} 
   \Contract[style={red,thick}]{m5}{B6} \Contract[vsep=-3pt,style={blue,thick}]{m6}{B5}
\]

\end{document}