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I like to use the stmaryrd symbol $\lightning$ for contradictions, but \require doesn't support stmaryrd nor wasysym (both packages have the lightning). Are there any MathJax alternatives?

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  • $\begingroup$ I don't recall ever seeing a lightning-bolt symbol used for a contradiction, so I suspect it will confuse a number of readers. Most common in my experience is $\, \Rightarrow\!\Leftarrow\ \ $ $\endgroup$ Commented Apr 15, 2015 at 18:27
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    $\begingroup$ It is common in our university, and it looks cool :) $\endgroup$
    – user459879
    Commented Dec 11, 2018 at 12:10

2 Answers 2

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MathJax has a \unicode extension, so you can specify the codepoint of a desired symbol per \unicode{x21af} (for the contradiction lightning) as in

$$\dotsc\implies 3 = 4\quad\unicode{x21af}.$$

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  • $\begingroup$ Thanks, that's what I hoped for. Playing around with its size now. $\endgroup$
    – AlexR
    Commented Apr 15, 2015 at 8:50
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    $\begingroup$ I found $\unicode{x26A1}$ and $\unicode{x2607}$ also. $\endgroup$
    – robjohn Mod
    Commented Apr 15, 2015 at 8:50
  • $\begingroup$ Testing huge: $\huge\unicode{x21af}$ Large $\Large\unicode{x21af}$ $\endgroup$
    – AlexR
    Commented Apr 15, 2015 at 8:52
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    $\begingroup$ You can even simply put the character verbatim in the code: $1+1 = 2 ↯.$ It also works for the size: $\Huge ↯$. $\endgroup$ Commented Apr 15, 2015 at 9:01
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Honestly, I generally think that words are better than symbols here, especially when there is no universal "contradiction" symbol. So I'd suggest to go with something to the effect of

..., this is a contradiction.

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    $\begingroup$ Sometimes I find this too clunky. Especially when giving a one-line proof for a small remark via contradiction. (If $X$ were $Y$, then $1=0$ \lightning) reads more compact and can be ignored more easily by the eye if the reader isn't interested. $\endgroup$
    – AlexR
    Commented Apr 15, 2015 at 8:24
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    $\begingroup$ I get what you're saying. If it's a contradiction to a basic mathematical fact, I'd probably leave out any further indication and rely on the reader being able to discern that a contradiction has been found. (I.e., if I'm quickly trying to convince someone that every foo is bar, I might just leave a parenthetical remark to the effect of "otherwise $4$ is prime".) In other cases I generally prefer to point out what is being contradicted (some assumption, some more advanced fact, etc.). $\endgroup$
    – user642796
    Commented Apr 15, 2015 at 8:45
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    $\begingroup$ I rarely see the lightning symbol. I have usually seen "$\rightarrow\leftarrow$" or "$\perp$" as a contradiction symbol. $\endgroup$
    – robjohn Mod
    Commented Apr 15, 2015 at 8:57
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    $\begingroup$ @robjohn: You should watch The Flash. :-) $\endgroup$
    – Asaf Karagila Mod
    Commented Apr 15, 2015 at 9:18
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    $\begingroup$ @AsafKaragila: when I do, I see a blue spot for several minutes :-) $\endgroup$
    – robjohn Mod
    Commented Apr 15, 2015 at 9:31
  • $\begingroup$ I'm a first year maths student and have already seen the lightning symbol and another symbol, ※ used by teaching staff. $\endgroup$
    – lukeuser
    Commented Oct 22, 2017 at 2:44
  • $\begingroup$ This is not an answer to the question. $\endgroup$
    – Nearoo
    Commented Nov 12, 2018 at 17:54

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