# amstex: operator definition error

I can't understand why the following code give me an error

\input amstex.tex
$\mathop{\rm arctg}(x)$
\bye


Thus, the input does not work with my macros that include the use of \rm, \mathop, etc., instructions.

Error:

! AmS-TeX error: Use \rm only in text.
\err@ #1->\errmessage {AmS-TeX error: #1}
\rm ...\Err@ {Use \string \rm \space only in text}
\else \tenrm \fi
l.3 $\mathop{\rm arctg}(x)$

• With amstex you should use \roman, \italic, \bold or \slanted for the corresponding fonts in math mode; so \mathop{\roman{arctg}} would work, although \operatorname{arctg} is better. As an aside, I know that "arctg" is traditional in Italy, but "arctan" is the international name (and the one defined by ISO regulations). May 15 '13 at 20:41
• @egreg: I can't resist mentioning that "arctg" is traditional in France too, but hand-held calculators have initiated thirty years ago a slow but constant decline, much to my dispair.
– user4686
May 15 '13 at 21:27
• @jfbu I prefer "arctan", to be honest. And also "sin", while in Italy it's traditional to use "sen". May 15 '13 at 21:30
• @egreg it's not "arctan" which I dislike but the way it has raised to pre-eminence. I must be deeply conservative. Actually I am, I guess.
– user4686
May 15 '13 at 21:39
• @egreg "sen" has a more sensual look than "sin"... ok, sorry, and apologies to the OP for this digression.
– user4686
May 15 '13 at 21:41

for a number of reasons, ams-tex handles fonts differently in math than in text, and \rm has been defined to be a text-only font, resulting in the error
! AmS-TeX error: Use \rm only in text.

$\operatorname{arctg}(x)$
`