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Possible Duplicate:
Define additional math operators to be typeset in roman

If I want to use the function sin in LaTeX, I write $\sin x$ or $\sin(x+1)$. If I want to use the function arctanh, I can't use $\arctanh x$:

! Undefined control sequence.
<argument> ...\ (\cos x)' &= -\sin x \\ (\arctanh 
                                                  x)' &= \frac {1}{\sqrt {1 ...
l.74 \end{align*}

? 
! Emergency stop.

What should I use instead for arctanh?

I thought of $\text{arctanh} x$, but this doesn't leave a space after arctanh. Additionally, it seems to be semantically incorrect, as arctanh is not "only" text.

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marked as duplicate by egreg, Torbjørn T., Garbage Collector, Marco Daniel, clemens Sep 16 '12 at 10:17

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You could also use \atanh{x} of the physymb package. It gives tanh^-1. – Harold Cavendish Sep 16 '12 at 9:55
2  
arctanh is semantically wrong. The right word is artanh, where "ar" is the abbreviation for area and not for arc. – Luigi Sep 16 '12 at 11:25
up vote 11 down vote accepted

\text is not well suited for this, use

\usepackage{amsmath}
\DeclareMathOperator\arctanh{arctanh}

then \arctanh behaves like \tan and friends

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3  
typo: MathOperator not MahOperator – David Carlisle Sep 16 '12 at 10:06

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