### 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 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”.

The amsopn command `\``operatorname` allows you to introduce ad hoc operators into your mathematics; as with `\``DeclareMathOperator` there’s a starred version `\``operatorname*` for sub- and superscripts in the limits position.

(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 (gzipped tar, browse)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=newfunction