# Is there no good way to write \not\iff?

It always comes out funky, and I'd like to avoid \nLeftrightarrow. Is there any good way to negate the longer \iff double implication arrow?

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Please ignore my request to close.... It was a misake... –  Peter Grill Oct 5 '12 at 3:11
haha no worries man - thanks a ton! –  Sean Allred Oct 5 '12 at 3:24
What would $A \not\iff B$ even mean mathematically? It seems to me that saying that exactly one of A or B holds is simpler and clearer. –  lhf Oct 5 '12 at 12:09

You can adapt the solutions form \Rightarrow vs. \implies, and "does not imply" symbol:

## Code:

\documentclass{article}
\usepackage{amsmath}
\usepackage{centernot}

\newcommand{\notiff}{%
\mathrel{{\ooalign{\hidewidth$\not\phantom{"}$\hidewidth\cr$\iff$}}}}

\begin{document}
$A \notiff B$

$A \centernot\iff B$
\end{document}

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+1 egreg's favourite tool :) –  cmhughes Oct 5 '12 at 3:17
I didn't even know something like centernot existed... one wonders why this isn't default behavior! Thanks!! –  Sean Allred Oct 5 '12 at 3:23
@vermiculus It isn't the default because those symbols should not be used under any circumstance. They mean nothing, IMHO. –  egreg Oct 5 '12 at 6:33
@egreg, in many cases, it is clearer to negate an operator than to negate a predicate (especially when writing for other students). –  Sean Allred Oct 10 '12 at 4:28
@vermiculus You're stating a "metanontheorem". Yesterday I was talking about the noncommutativity of matrix multiplication where one cannot say "AB≠BA". Most usages of "doesn't imply" fall under the same case. –  egreg Oct 10 '12 at 9:15