I tried to get the trig function cosec in MathJax, but it comes out as $\cosec$. In contrast, it works perfectly well for sec: $\sec$. Of course, one can always use \operatorname{cosec} as a workaround ($\operatorname{cosec})$; but this seems clumsy. The shorter form csc works: $\csc$. Perhaps the designers of MathJax simply don't like cosec and think that we should all switch to csc.

  • $\begingroup$ This is not a bug wrt Stack Exchange software, it is a feature of mathjax, over which Stack Exchange has no control. Mathjax is not unique to some stack exchange sites. $\endgroup$ – amWhy Apr 2 '20 at 17:05
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    $\begingroup$ It also recognizes $\sinh x$, but not $\sech x$. $\endgroup$ – Gerry Myerson Apr 2 '20 at 21:44
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    $\begingroup$ Related (a bit): Request for Hyperbolic $ \sech \,x $ inclusion in MathJax. $\endgroup$ – Martin Sleziak Apr 3 '20 at 4:51

No, you're right that mathjax doesn't recognize $\cosec(x)$: $\cosec(x)$, but it recognizes $\csc(x)$: $\csc(x)$ for use when you want to denote cosecant(x). It is likely due to the fact that \sin(x), \cos(x), \tan(x), \sec(x), \cot(x) all have three-letter operator-names, and so the choice of operator for cosecant, in mathjax, similarly has a three-letter operator: \csc(x).

Even though one can speak of $\csc(x) = \frac 1{\sin x}$, mathjax does not recognize $\arccsc(x)$: $\arccsc(x)$, though it recognizes $\csc^{-1}(x)$: $\csc^{-1}(x)$. But nor does it recognize $\arcsec$: $\arcsec$, nor $\arccot$: $\arccot$.

But we do have $\arcsin, \arccos, \arctan$: $\arcsin, \arccos, \arctan$.

Now, a work around, if you're needing to use cosecant a lot in one post, is to write \DeclareMathOperator{cosec}{cosec} before needing to use cosec. Then, you can use it as you like, as you expected you could, by writing \cosec(x). I display the results immediately below. You can click on "edit" to see that it does indeed work.




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