\begin{equation} len(c) = length(c,rdfs:subClassOf))
\end{equation} 
Danke!
\begin{equation} len(c) = length(c,rdfs:subClassOf))
\end{equation} 
\mathit{len}(c) = \mathit{length}(c,\mathit{rdfs{:}subClassOf})\documentclass{article}
\makeatletter
\newcommand\len{\mathop{\operator@font len}}
\newcommand\length{\mathop{\operator@font length}}
\makeatother
\begin{document}
\begin{equation}
  \len(c) = \length(c,\mathit{rdfs{:}subClassOf}) 
\end{equation}
\end{document}\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
  \text{len}(c) = \text{length}(c,\text{rdfs:subClassOf})
\end{equation}
\end{document}