\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,2) node(TTT){$T^3$} (2,2) node(TTP){$T^2$};
  \path (0,0) node(PTT){$T^2$} (2,0) node(T){$T$};
  \draw[->] (TTT) -- node[above]{$T\mu$} (TTP);
  \draw[->] (TTT) -- node[left]{$\mu T$}(PTT);
  \draw[->] (TTP) -- node[right]{$\mu$} (T);
  \draw[->] (PTT) -- node[below]{$\mu$} (T);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: