Negation of nleftrightarrow?

Question

If I use $\nleftrightarrow$ from the amssymb package, I get a negated double-headed arrow. So far, so good.

Unfortunately, the $\nleftrightarrow$ seems to have smaller arrow heads then the $\leftrightarrow$, which does not look nice if I want to use one symbol for the negation of the other.

What is the non-negated equivalent of the $\nleftrightarrow$?

Answer 1

If you are able to use Lua- or XeLaTeX, you may want to use the package unicode-math which got this fixed. The default font here is Latin Modern Math as would be standard in 'normal' LaTeX.

% arara: lualatex

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

\begin{document}
$A\leftrightarrow B\nleftrightarrow C$
\end{document}

Here are the possible fonts and the unicode presented in the manual of unicode-math:

Answer 2

Taking the suggestion of skpblack, but limiting the importation to just those two glyphs from mathabx (since that package changes many, many glyphs), we have here denoted those two imported glyphs as \varleftrightarrow and \nvarleftrightarrow. All other math glyphs are as defined by amssymb or LaTeX, itself.

\documentclass{article}
\usepackage{amssymb}
% Setup the matha font (from mathabx.sty)
\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
      <5> <6> <7> <8> <9> <10> gen * matha
      <10.95> matha10 <12> <14.4> <17.28> <20.74> <24.88> matha12
      }{}
\DeclareSymbolFont{matha}{U}{matha}{m}{n}

% Define a subset character from that font (from mathabx.dcl)
% to completely replace the \subset character, you can replace
% \varsubset with \subset

\DeclareMathSymbol{\varleftrightarrow}{3}{matha}{"D8}
\DeclareMathSymbol{\nvarleftrightarrow}{3}{matha}{"DC}
\begin{document}
\texttt{amssymb} left-right arrow and negation
\[
A \leftrightarrow B \nleftrightarrow C
\]

\texttt{mathabx} left-right arrow and negation
\[
A \varleftrightarrow B \nvarleftrightarrow C
\]
\end{document}