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Before anyone slates me for this, I have checked detexify and texdoc symbols. Indeed, the latter led me to the following phrase (3rd paragraph of Section 3):

Although there have been many requests on comp.text.tex for a contradiction symbol, the ensuing discussion invariably reveals innummerable ways to represent contradiction in a proof ... Because of the lack of notational consensus, it is probably better to spell out "Contradiction!" than to use a symbol for this purpose.

Normally, I'd agree with this sage advice. Two things prevent me from doing so:

  1. In a presentation, brevity is the soul of comprehension.

    That is to say, it is better still to have a funny symbol and to say "Thus we have a contradiction" than to have the long word "Contradiction!" and still say "Thus we have a contradiction".

  2. The ellipsis in the above quote lists various symbols that the discussion on comp.text.tex presumably discussed. None of them is correct. The correct contradiction symbol is (something like):

    \ \/ /
     \/\/
     /\/\
    / /\ \
    

    That is, four diagonal lines, two in each direction. It should also be a bit larger than a "regular" symbol, perhaps more like a \prod or \sum.

So, my question: is there a font with this symbol?

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8  
I've never heard of a correct contradiction symbol. Sometimes I use \Lightning from marvosym. –  egreg Sep 12 '11 at 11:04
4  
Do you mean the unicode symbol "⨳" (U+2A33, "SMASH PRODUCT") (mentioned in the Wikipedia entry for "Proof by contradiction)? –  diabonas Sep 12 '11 at 11:34
2  
@Andrew -- what is the smash product then? since i was the person responsible for communicating the information to the unicode technical committee, based on resources from a number of (usually reliable) sources, and the symbol in the unicode charts matches those sources, i'm really interested in this. if it's not correct, i should be able to at least get the image corrected and a note added in the unicode charts, though it's probably not possible to get changes made elsewhere. (contact me off-line, please, if you want to learn the sources.) –  barbara beeton Sep 12 '11 at 13:20
3  
@barbara: I've now checked with one of my more ... experienced ... colleagues. He says that this symbol was used for the smash product in the days of G. Whitehead, but hasn't been used for a long time and that nowadays the wedge (logical or) is used. He also said that the old symbol was sometimes more of a "sharp" sign (though that may have been more due to what fonts printers had available). –  Loop Space Sep 12 '11 at 17:57
2  
@Andrew -- thanks for checking. i will make a note to submit a clarification to the unicode technical committee in this regard, to insert a note at U+2A33 that this symbol is now out of favor, having been superseded by U+2227 (correct this if it's one of the other similar-shaped objects). it would also be helpful to have an example; please send citation that i can refer to. –  barbara beeton Sep 12 '11 at 21:14

4 Answers 4

up vote 11 down vote accepted

The Unicode symbol "⨳" (U+2A33, it is called "SMASH PRODUCT" for some mysterious reasons) you are looking for is available with modern TeX engines (XeTeX, LuaTeX): You'll have to load the unicode-math package and an appropiate OpenType math font such as XITS Math, then you can access it as \smashtimes.

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{XITS Math}
\begin{document}
    $\smashtimes$
\end{document}
share|improve this answer
    
This is great and all but requires the installation of XITS Math, which is not part of TeX Live. Translation: you've got to install fonts from github and make fontconfig aware of them. –  g33kz0r Apr 14 '12 at 22:37
2  
@g33kz0r XITS Math is part of TeX Live, which you can see on the package info page. I tested it with an unmodified version of TeX Live 2011 and it works perfectly fine out of the box. –  diabonas Apr 15 '12 at 19:35
    
pastie.org/3794801 –  g33kz0r Apr 15 '12 at 22:17
    
@g33kz0r I see you're using the development version of TeX Live 2012, so maybe it's a bug/incompatibility of this (yet unstable) version. A hint may be the error message <code>kpathsea: Invalid fontname XITS Math/ICU', contains ' '. You can try using \setmathfont{xits-math.otf} to see if this works better, or switch to LuaLaTeX which uses a different method to find the fonts. –  diabonas Apr 16 '12 at 7:30

My attempt, based on diabonas' answer and flying sheep's comment:

\documentclass{article}
\usepackage{tikz}

\newcommand{\contradiction}{%
\begin{tikzpicture}[rotate=45,x=0.5ex,y=0.5ex]
\draw[line width=.2ex] (0,2) -- (3,2) (0,1) -- (3,1) (1,3) -- (1,0) (2,3) -- (2,0);
\end{tikzpicture}
}

\begin{document}

Contradiction? \contradiction

\end{document}

Contradiction

Bear with me, I'm a TikZ newbie. =P

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2  
I used this approach, except with a thinner line width of .1ex, and it looks great! It's simple, concise, and make great use of TikZ, which I am likely already using in any given document. Thanks for the tip! –  Tim Parenti Jul 31 '12 at 18:25

I don't think there's a "correct" symbol for contradiction. I use the symbol you describe (in my handwritten notes) for things along the lines of "contradiction" or "this is obviously wrong" or "aargh I've made a bad mistake somewhere".

If you \usepackage{mathabx}, you can get a nice large cross symbol which you can use to construct the double-cross: something along the lines of

\mbox{\rlap{$\displaystyle\bigtimes$}{$\displaystyle\,\bigtimes$}}

would work.

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or draw it yourself for maximum control: \usepackage{tikz} and then \def\contra{\tikz[baseline, x=1ex, y=.7em, line width=.7pt]\draw (0,0) -- (1,1) (0,1) -- (1,0) (.5,0) -- (1.5,1) (0.5,1) -- (1.5,0);} –  flying sheep Sep 12 '11 at 11:27
    
Nice! My favourite, with only a minor tweak, is \usepackage{tikz} and then \def\contra{\tikz[baseline, x=0.22em, y=0.22em, line width=0.032em]\draw (0,2.83)--(2.83,0) (0.71,3.54)--(3.54,0.71) (0,0.71)--(2.83,3.54) (0.71,0)--(3.54,2.83);} –  Harry Macpherson Aug 23 '12 at 13:33

Here's a macro that kludges this symbol out of four \times signs:

\newcommand{\contradiction}{{\hbox{%
    \setbox0=\hbox{$\mkern-3mu\times\mkern-3mu$}%
    \setbox1=\hbox to0pt{\hss$\times$\hss}%
    \copy0\raisebox{0.5\wd0}{\copy1}\raisebox{-0.5\wd0}{\box1}\box0
}}}

enter image description here

This macro requires no additional packages or special fonts; everything it uses is plain vanilla LaTeX.

An explanation of how this works: The second line (beginning with \setbox0) creates a box containing a single \times sign and stores this box in register 0; the \mkern commands add a bit of negative space on either side of the \times so that the left and right sides of the box are flush with the edges of the \times symbol. The next line (beginning with \setbox1) creates a similar box in register 1, except that this box has width 0, with the \times symbol centered horizontally (\hss stands for "horizontal stretch or shrink"; putting it on both sides achieves the centering effect). So here the \times symbol actually extends outside the zero-width box, equally far on both sides. The fourth line copies the contents of the box in register 0 to the output, then copies the contents of the box in register 1 to the output after raising it by half of the width of the box in register 0 (i.e., half of the width of the \times sign), then moves the box in register 1 to the output after lowering it by half of the width of the box in register 0, then moves the box in register 0 to the output.

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The display of the output of your command is not proper. Could you review it and update your answer? –  azetina Jun 12 '12 at 20:24
    
I don't understand. What about it needs to be fixed? Here's how it looks in my test document: oi46.tinypic.com/2crx37o.png –  Brian Kell Jun 12 '12 at 20:33
    
I understand but the code needs to be refined. Try to compile with out any text in front of the command and you will see what I mean. –  azetina Jun 12 '12 at 20:40
    
Oh, I see. You're right. I've added an \hbox around the whole thing. Does that solve the problem? (By the way, there is probably some more LaTeX-esque way to do some of these things, like maybe LaTeX prefers to use \mbox instead of \hbox for some reason. I come from the Plain TeX world, and I don't understand LaTeX sometimes.) –  Brian Kell Jun 12 '12 at 20:46
    
I actually like this approach using plain TeX. How and what did you use to learn it? Could you provide links or documentation on how to learn them before me asking here? –  azetina Jun 12 '12 at 20:54

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