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