With the help of some code that I found here I have managed to produce the arrows I was in need of, namely:
\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
% new arrow tip
\newdimen\arrowsize
\pgfarrowsdeclare{arcs'}{arcs'}{...}
{
\arrowsize=0.5pt
\advance\arrowsize by .5\pgflinewidth
\pgfsetdash{}{0pt} % do not dash
\pgfsetroundjoin % fix join
\pgfsetroundcap % fix cap
\pgfpathmoveto{\pgfpoint{-4\arrowsize}{4\arrowsize}}
\pgfpatharc{180}{270}{4\arrowsize}
\pgfpatharc{90}{180}{4\arrowsize}
\pgfusepathqstroke
}
% new arrows for IMPLICATION, COIMPLICATION, and some close relatives
\makeatletter
\newbox\xrat@below
\newbox\xrat@above
\newcommand{\imparrow}[2][]{%
\setbox\xrat@below=\hbox{\ensuremath{\scriptstyle #1}}%
\setbox\xrat@above=\hbox{\ensuremath{\scriptstyle #2}}%
\pgfmathsetlengthmacro{\xrat@len}{max(\wd\xrat@below,\wd\xrat@above)+.6em}%
\mathrel{\tikz [->,>=arcs',baseline=-.5ex,line width=.6pt]
\draw (0,0) -- node[below=-2pt] {\box\xrat@below}
node[above=-2pt] {\box\xrat@above}
(\xrat@len,0) ;}}
\makeatother
\newcommand{\impl}{\mathrel{\imparrow[\phantom{\text{--}}]{}}}
\newcommand{\invimpl}{\mathrel{\reflectbox{$\imparrow[\phantom{\text{--}}]{}$}}}
%
\makeatletter
\newbox\xrat@below
\newbox\xrat@above
\newcommand{\coimparrow}[2][]{%
\setbox\xrat@below=\hbox{\ensuremath{\scriptstyle #1}}%
\setbox\xrat@above=\hbox{\ensuremath{\scriptstyle #2}}%
\pgfmathsetlengthmacro{\xrat@len}{max(\wd\xrat@below,\wd\xrat@above)+.65em}%
\mathrel{\tikz [>-,>=arcs',baseline=-.5ex,line width=.6pt]
\draw (0,0) -- node[below=-2pt] {\box\xrat@below}
node[above=-2pt] {\box\xrat@above}
(\xrat@len,0) ;}}
\makeatother
\newcommand{\coimpl}{\mathrel{\;\coimparrow[\phantom{\text{}}]{}}}
\newcommand{\invcoimpl}{\mathrel{\reflectbox{$\;\coimparrow[\phantom{\text{}}]{}$}}}
%
\makeatletter
\newbox\xrat@below
\newbox\xrat@above
\newcommand{\biimparrow}[2][]{%
\setbox\xrat@below=\hbox{\ensuremath{\scriptstyle #1}}%
\setbox\xrat@above=\hbox{\ensuremath{\scriptstyle #2}}%
\pgfmathsetlengthmacro{\xrat@len}{max(\wd\xrat@below,\wd\xrat@above)+.3em}%
\mathrel{\tikz [>-<,>=arcs',baseline=-.5ex,line width=.6pt]
\draw (0,0) -- node[below=-2pt] {\box\xrat@below}
node[above=-2pt] {\box\xrat@above}
(\xrat@len,0) ;}}
\makeatother
\newcommand{\biimpl}{\mathrel{\,\biimparrow[\phantom{\text{}}]{}\,}}
%
\makeatletter
\newbox\xrat@below
\newbox\xrat@above
\newcommand{\cobiimparrow}[2][]{%
\setbox\xrat@below=\hbox{\ensuremath{\scriptstyle #1}}%
\setbox\xrat@above=\hbox{\ensuremath{\scriptstyle #2}}%
\pgfmathsetlengthmacro{\xrat@len}{max(\wd\xrat@below,\wd\xrat@above)+.72em}%
\mathrel{\tikz [<->,>=arcs',baseline=-.5ex,line width=.6pt]
\draw (0,0) -- node[below=-2pt] {\box\xrat@below}
node[above=-2pt] {\box\xrat@above}
(\xrat@len,0) ;}}
\makeatother
\newcommand{\cobiimpl}{\mathrel{\cobiimparrow[\phantom{\text{-}}]{}}}
\begin{document}
\LARGE
$\alpha\to\beta$ / $\alpha\Leftrightarrow\beta$\bigskip
% Implication
$\alpha \impl \beta$
% Converse implication
$\alpha \invimpl \beta$
% Coimplication
$\alpha \coimpl \beta$
% Converse coimplication
$\alpha \invcoimpl \beta$
$\alpha \biimpl \beta$
$\alpha \cobiimpl \beta$
\end{document}
producing the output
I am now having trouble to produce double-line versions of these same arrows. Could anyone help?
