\
mathop
command, as in:
\newcommand{\diag}{\mathop{\mathrm{diag}}}
Subscripts and superscripts on
\
diag
will be placed below and
above the function name, as they are on
\
lim
. If you want your subscripts and superscripts always placed
to the right, do:
\newcommand{\diag}{\mathop{\mathrm{diag}}\nolimits}
AMSLaTeX (in its amsopn package, which is automatically loaded by amsmath) provides a command
\
DeclareMathOperator
that takes does the same job as the first
definition above. To create our original \
diag
command, one would
say:
\DeclareMathOperator{\diag}{diag}
\
DeclareMathOperator*
declares the operator always to have its
sub- and superscripts in the
“\
limits
position”.
\
operatorname
allows you to
introduce ad hoc operators into your mathematics, so
\
[
\
operatorname{foo}
(bar)
\
]
typesets the same as
\DeclareMathOperator{\foo}{foo} ... \[ \foo(bar) \]
As with \
DeclareMathOperator
there’s a starred version
\
operatorname*
for sub- and superscripts in the limits position.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=newfunction