# Arrow under interval

How to produce an arrow under an interval in order to indicate that the two elements may have to be reversed?

For instance I would like to typeset [x, y] with a double-headed arrow between x and y.

Here is a sketch of the desire output:

I am using AMS packages.

Since this is tagged tikz-arrows, I am wondering if you are looking for something like this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{tikzmark,arrows.meta,bending}
\begin{document}
$[\tikzmarknode{x}{x},\tikzmarknode{y}{y}]\qquad [\tikzmarknode{Sn}{S_n},\tikzmarknode{Sn+1}{S_{n+1}}] \begin{tikzpicture}[overlay,remember picture,>={Stealth[bend]}] \draw[<->] (x.south|-y.south) to[out=-70,in=-110] (y.south); \draw[<->] (Sn.south|-Sn+1.south) to[out=-70,in=-110] (Sn+1.south); \end{tikzpicture}$
\end{document}


• It is a bit verbose but it does the job – user152057 Dec 27 '19 at 12:23

Here are some possibilities with the curved arrows of mathabx, without having to load the package, and \underset/ _overset or with stackengine:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools, amssymb}
\usepackage[usestackEOL]{stackengine}

\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{%
<-6> mathb5
<6-7> mathb6
<7-8> mathb7
<8-9> mathb8
<9-10> mathb9
<10-12> mathb10
<12-> mathb12 }{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}

\DeclareMathSymbol{\abxcurvearrowleft}{\mathrel}{mathb}{"F0}
\DeclareMathSymbol{\abxcurvearrowright}{\mathrel}{mathb}{"F1}
\DeclareMathSymbol{\curvearrowleftright}{\mathrel}{mathb}{"F2}
\DeclareMathSymbol{\curvearrowbotleft}{\mathrel}{mathb}{"F3}
\DeclareMathSymbol{\curvearrowbotright}{\mathrel}{mathb}{"F4}
\DeclareMathSymbol{\curvearrowbotleftright}{\mathrel}{mathb}{"F5}

\begin{document}

$[ \underset{\curvearrowbotleftright}{a, b}] \qquad [ \overset{\curvearrowleftright}{a, b}]\qquad [ \overset{\abxcurvearrowleft}{ \underset{\curvearrowbotright}{a, b}}]$%

\setstackgap{S}{1pt}%
\setstackgap{L}{7pt}
$\stackMath [ \Shortunderstack{a, b\\\curvearrowbotleftright}] \qquad [ \Shortstack{\curvearrowleftright\\a, \vphantom{i}\smash[t]{b}}]\qquad [ \Vectorstack{\abxcurvearrowleft\\ a, b \\ \curvearrowbotright}]$%

\end{document}


Using \underset from amsmath and \longleftrightarrow:

\documentclass[border=3.14]{standalone}

\usepackage[]{amsmath}

\begin{document}
$\underset{\longleftrightarrow}{[x,y]}$
\end{document}


If you think that this arrow is too long, you might use the normal \leftrightarrow:


\documentclass[border=3.14]{standalone}

\usepackage[]{amsmath}

\begin{document}
$\underset{\leftrightarrow}{[x,y]}$
\end{document}