UK TeX FAQ -- question 166

### Defining a new log-like function in LaTeX

Use the `\mathop` command, as in:

      \newcommand{\diag}{\mathop{\mathrm{diag}}}

Subscripts and superscripts on `\diag` will be placed 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) provides a command `\DeclareMathOperator` that satisfies the requirement.

(It should be noted that "log-like" was reportedly a joke on Lamport's part; it is of course clear what was meant.)

`amsopn.sty`
  In the AMSLaTeX distribution macros/latex/required/amslatex.tar.gz