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