A short hyphen in a math formula

Here are three suggestions. The first is my own, and the second and third are adapted from (http://www.logic.at/staff/salzer/etc/mhyphen/). Note that the first and second both output "math italic" text, but the third outputs "upright" text.

\documentclass{article}
\begin{document}
\newcommand\rnumber{\mathop{\mbox{$r$-$\mathit{number}$}}}
\[
\rnumber(5) = 120
\]
\end{document} 

\documentclass{article}
\usepackage{amsmath}
\mathchardef\mhyphen="2D % Define a "math hyphen"
\begin{document}
\newcommand\rnumber{\mathop{r\mhyphen number}}
\[
\rnumber(5) = 120
\]
\end{document} 

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\newcommand\rnumber{\operatorname{r-number}}
\[
\rnumber(5) = 120
\]
\end{document} 

LyX users should put the definitions (\usepackage, \newcommand, \mathchardef, etc.) in Document -> Settings -> LaTeX Preamble, and the usage (e.g. \rnumber(5) = 120) inside math-mode (Ctrl+M).

I think this is simpler.

$r{\text -}number(5)=120$

My first shot would be \DeclareMathOperator; but it depends on the meaning of "r": if it represents a math variable it should be in italics. I'll show both possibilities, take your pick.

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\rnumberA}{r-number}
\DeclareMathOperator{\rnumberB}{\mathnormal{r}-number}

\begin{document}

$\rnumberA(2)$

$\rnumberB(2)$

\end{document}