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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
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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
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1 Answer 1

up vote 5 down vote accepted

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

enter image description here

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
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@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
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