# [texhax] Something funny about \mathop (fwd)

Michael Barr barr at math.mcgill.ca
Mon Jul 26 14:42:57 CEST 2010

I have answered my own question below.  The answer is found in point 13 of
Appendix G of The TeXBook bottom of page 443 and top of 444 in my
(the first) edition.  Although I didn't follow the technical details it is
clear that TeX will lower a single character \mathop to center it along
the horizontal axis.  It is not clear why an empty pair of braces defeats
this mechanism but it does.  It is clear that this was intended for things
like integral signs and not single alphabetic characters, but it doesn't
distinguish between them.  Curiously, this is not indexed (not under
\mathop, in any case) in the TeXBook.

Michael Barr

---------- Forwarded message ----------
Date: Mon, 26 Jul 2010 06:26:25 -0400 (EDT)
From: Michael Barr <barr at math.mcgill.ca>
To: texhax at tug.org

This file illustrates the "something funny"
\documentclass[12pt]{article}
\begin{document}

\def\I{\mathop{\rm I}}
\def\II{\mathop{\rm II}}
This is an example $\I\II\I\II$
\def\I{\mathop{\rm{}I}}
This is an example $\I\II\I\II$

\end{document}

The two definitions of \I ought to give the same results, but don't.  The
lowering doesn't occur with \II.

Michael Barr