173

I apparently can't see the difference in a compilation (pdflatex) between \mathrm and \operatorname. So, as in the title the question is: is there a difference? Which one is preferable in what occasions?

201

There are many differences. The main one is in the fact that \mathrm{xyz} behaves like an ordinary letter, while \operatorname{xyz} behaves like function names such as \sin. Here's an illustration

$\sin x + \sin(x+y) + a\sin z$

$\mathrm{sin} x + \mathrm{sin}(x+y) + a\mathrm{sin}z$

enter image description here

where it's clear that the second line is wrongly typeset. Even if your "operator" requires parentheses after it, it should be \operatorname, as the third summand shows, where a thin space separates the coefficient from the operator.

Another subtle difference is in how some characters are interpreted in \mathrm and in \operatorname. Suppose you have an operator to be called "pre-norm", with a hyphen. Here's the example

$\operatorname{pre-norm}(\mathbf{v})$

$\mathrm{pre-norm}(\mathbf{v})$

enter image description here

and now it's clear what is to be used. Indeed \operatorname (and the same holds for macros defined with \DeclareMathOperator) treats in a special way punctuation symbols; \mathrm, instead, treats them as math symbols.

  • 1
    Can you please tell the full form of the rm in mathrm? Thank you! – Gaurang Tandon Feb 14 '18 at 5:12
  • 2
    @GaurangTandon rm stands for “roman”, that conventionally denotes the upright type used in the document. – egreg Feb 14 '18 at 8:59
35

\operatorname:

The argument is written in upright mode but with some additional space before and behind.

\mathrm:

It is like math mode (no spaces), but in upright mode. The font size isn't changed.

The following example shows the differences:

\documentclass[]{article}
\usepackage{amsmath}
\begin{document}
\[x\operatorname{foo}y\]
\[x\mathrm{foo}y\]
\end{document}

enter image description here

9

In other LaTeX environments \mathrm{xyz} is useful. In MathJax v 2.0

$ i \\, \mathrm{sinh} \; $

gave the equivalent output as $ i \sinh x $. MathJax did not recognize \operatorname.

  • 9
    \operatorname is now supported with the ASMmath.js TeX extension. – ChrisR Dec 4 '13 at 16:10

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