Implementing \lefttorightarrow from mathabx without importing the other symbols [duplicate]

Question

I am trying to implement this code taken from https://mathoverflow.net/q/64544/101848:

\usepackage{mathabx,epsfig}
\def\acts{\mathrel{\reflectbox{$\righttoleftarrow$}}}

so that G\acts X yields

The problem is that loading mathabx has the side effect of importing all symbols from mathabx. For example, it changes the default LaTeX \to, which is , to the symbol . A similar change happens to the default LaTeX \mapsto and it also changes the default LaTeX \in.

How could I have what the \acts command does without loading the rest of the mathabx symbols?

Answer 1

The importing code can be improved to use the font at any size. Also the definition of \acts can be improved to make a symbol that correctly scales in subscripts and superscripts.

\documentclass{article}
\usepackage{amsmath,graphicx}

\DeclareFontFamily{U}{mathb}{}
\DeclareFontShape{U}{mathb}{m}{n}{
     <-5.5> mathb5
  <5.5-6.5> mathb6
  <6.5-7.5> mathb7
  <7.5-8.5> mathb8
  <8.5-9.5> mathb9
  <9.5-11>  mathb10
  <11->     mathb12
}{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}
\DeclareMathSymbol{\lefttorightarrow}{3}{mathb}{"FC}
\DeclareMathSymbol{\righttoleftarrow}{3}{mathb}{"FD}

\makeatletter
\DeclareRobustCommand{\looparrow}[1]{%
  \mathrel{\mathpalette\looparrow@{#1}}%
}
\newcommand{\looparrow@}[2]{\reflectbox{$\m@th#1#2$}}
\makeatother

\newcommand{\acts}{\looparrow{\righttoleftarrow}}
\newcommand{\foo}{\looparrow{\lefttorightarrow}}

\begin{document}

$G\acts X$

$X\foo G$

$A_{G\acts X}$

\end{document}

Maybe you don't need the reverse symbol, but it comes almost for free.

Don't use \def and you'll be happier.

Answer 2

The ⟳ symbol (Clockwise gapped circle arrow) is also in unicode-math (and the legacy packages stix and stix2) as \cwgapcirclearrow. You can therefore use:

\documentclass{article}
\usepackage{unicode-math}

\setmathfont{STIX Two Math}

\begin{document}
\[ G \cwgapcirclearrow X \]
\end{document}

To import only this one symbol from a font that has it (such as STIX Two Math, XITS Math or New Computer Modern Math), you can use the range= option of \setmathfont:

\documentclass{article}
\usepackage{unicode-math}

\setmainfont{Erewhon}
\setmathfont{Erewhon Math}
\setmathfont{STIX Two Math}[ range = \cwgapcirclearrow,
                             Scale = MatchUppercase ]

\begin{document}
\[ G \cwgapcirclearrow X \]
\end{document}

Answer 3

Mimicking what is done in https://tex.stackexchange.com/a/14388/150120, I have come with the following code for the preamble which solves the problem:

\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{
      <5> <6> <7> <8> <9> <10> gen * mathb
      <10.95> mathb10 <12> <14.4> <17.28> <20.74> <24.88> mathb12
      }{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}

\DeclareMathSymbol{\righttoleftarrow}{3}{mathb}{"FD}

\def\acts{\mathrel{\reflectbox{$\righttoleftarrow$}}}

The six first lines of code are taken from the mathabx.sty file available at CTAN: https://www.ctan.org/tex-archive/fonts/mathabx/texinputs.

The "FD code for \righttoleftarrow was found looking for it in the mathabx.dcl file (also available in the last link).

I am posting this answer because the six first lines of code above are not exactly the same ones as those from the quoted TSE answer. The ones here are for mathb whereas the ones from the other TSE answer are for matha. I copied at first the code from the other TSE answer and it took me a while before realizing a little change was needed.