文字下方的箭头

Question

如何在 LaTeX 中输入以下内容？

Answer 1

这里有一个解决方案，它设置了一个名为的宏\placebelow，然后使用array来自\mathclap包的环境mathtools。

如果您希望使用较小的字体排版注释（“Z 中的附加内容”等），只需在下面的代码中更改#2为。\footnotesize #2

\documentclass{article}
\usepackage{mathtools} % for \mathclap macro
\usepackage{newtxtext,newtxmath}
\newcommand\placebelow[2]{\mathbin{%
     \begin{array}[t]{@{}c@{}} 
        #1 \\ \uparrow \\ \mathclap{\textbf{\textsf{#2}}}
     \end{array}}}
\begin{document}
\[
  f(x\placebelow{+}{Addition in Z}y)=[x+y]=[x]+[y]
 =f(x)\placebelow{+}{Addition in Z\textsubscript{6}}f(y)
\]
\end{document}

Answer 2

仅使用 TeX 原语的解决方案：

\def\type#1#2{\mathbin{\mathop{#1\kern0pt}%
   \limits_{\raise2.5ex\hbox to0pt{\hss\strut$\uparrow$\hss}\hbox to0pt{\hss#2\hss}}}}

$$
  f(x \type+{Addition in $Z$} y) = [x+y] = [x] + [y] = f(x) \type+{Addition in $Z_6$} f(y)  
$$

Answer 3

\underset这是一种使用(from amsmath) 和\bclap(from )的方法stackengine：

\documentclass{article}
\usepackage{amsmath}
\usepackage[usestackEOL]{stackengine} 

\begin{document}
%
\[ f(x \underset{\uparrow\bclap{\small\sffamily\bfseries Addition in Z}}{+} y) = [x + y] = [x] + [y] ) = f(x) \underset{\uparrow\bclap{\small\sffamily\bfseries Addition in Z\textsubscript{6}}}{+}f( y) \]%


\end{document} 

Answer 4

常规解决方案是tikzmarknode

\documentclass{article}
\usepackage{mathtools} % for \mathclap macro
\usepackage{newtxtext,newtxmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\begin{document}
    \[
    f(x\tikzmarknode{a}{+}y)=[x+y]=[x]+[y]
    =f(x){+}f(y)
    \]
    
    \begin{tikzpicture}[overlay,remember picture]

    \draw[%
          <-,
          very thin, 
          >=stealth, 
          red!50
          ]% 
    (a.south)--++(0pt,-3em) node [below](X) {X};
    \end{tikzpicture}
    
\end{document}