How do I write logical iff symbol?

Question

I'm currently hearing a lecture about formal systems (a computer science lecture).

The lecturer makes a symbol that is written like overlying < and >:

Every time when he draws it, he says "genau dann wenn" (German for "if, and only if").

How do I crate this symbol with LaTeX?

Answer 1

And here is more simple definition than @egregs ooalign:

\def\gdw{\mathrel{{>}\mkern-13mu{<}}}

$A \gdw B$

And it is optical better:

Answer 2

A simple application of \ooalign:

\documentclass{article}

% Simple version if you don't need it in sub/superscripts
%\newcommand\gdw{\mathrel{\ooalign{$<$\cr$>$\cr}}}

% Fuller version
\makeatletter
\newcommand{\gdw}{\mathrel{\mathpalette\@gdw@\relax}}
\newcommand{\@gdw@}[2]{\ooalign{$\m@th#1<$\cr$\m@th#1>$\cr}}
\makeatother

\begin{document}
$X \gdw Y_{\gdw}$
\end{document}

See this answer for a quick course on \ooalign. What's the advantage of this complicated looking solution over a seemingly simpler “print >, back up and print <”? That you don't need to guess the width of the symbol, which might change with the font used; with \ooalign you don't run the risk of having to compute the width.

If you want to make the inner rhombus smaller, you can add some pushing:

\documentclass{article}

\makeatletter
\newcommand{\gdw}{\mathrel{\mathpalette\@gdw@\relax}}
\newcommand{\@gdw@}[2]{%
  \ooalign{$\m@th#1\@gdw@push<$\cr$\m@th#1>\@gdw@push$\cr}}
\newcommand{\@gdw@push}{\mkern2mu}% adjust to suit
\makeatother

\begin{document}
$X \gdw Y_{\gdw}$
\end{document}

Another possibility:

\documentclass{article}
\usepackage{mathtools}

\newcommand{\gdw}{%
  \mathrel{\mathrlap{>}}% print > with zero width
  \mathrel{\mkern2mu}% some small spacing
  <% print the <
}

\begin{document}
$X \gdw Y_{\gdw}$
\end{document}

This produces exactly the same output as the last one (with the 2mu to be adjusted to suit). It is simpler in its aspect, because it uses \mathpalette internally. It exploits the fact that TeX inserts no space between consecutive relation atoms.

Answer 3

This is the unicode U+2AA4 named "GREATER-THAN OVERLAPPING LESS-THAN". Here, you may see, which fonts support this symbol and here, which fonts on your system do provide it.

The MWE requires Lua- or XeLaTeX. It just shows two fonts I found on my PC. Of course, you should define your preferred version in a \mathrel-command.

% arara: lualatex

\documentclass{article} 
\usepackage{fontspec}

\begin{document} 
\fontspec{Asana Math}\symbol{"2AA4}
\fontspec{XITS Math}\symbol{"2AA4}
\end{document}