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?

• 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\ \$ – Bill Dubuque Apr 15 '15 at 18:27
• It is common in our university, and it looks cool :) – Wesley Strik Dec 11 '18 at 12:10

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

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

• Thanks, that's what I hoped for. Playing around with its size now. – AlexR Apr 15 '15 at 8:50
• I found $\unicode{x26A1}$ and $\unicode{x2607}$ also. – robjohn Apr 15 '15 at 8:50
• Testing huge: $\huge\unicode{x21af}$ Large $\Large\unicode{x21af}$ – AlexR Apr 15 '15 at 8:52
• You can even simply put the character verbatim in the code: $1+1 = 2 ↯.$ It also works for the size: $\Huge ↯$. – Najib Idrissi Apr 15 '15 at 9:01

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

• 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. – AlexR Apr 15 '15 at 8:24
• 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.). – user642796 Apr 15 '15 at 8:45
• I rarely see the lightning symbol. I have usually seen "$\rightarrow\leftarrow$" or "$\perp$" as a contradiction symbol. – robjohn Apr 15 '15 at 8:57