\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,-2) node(B){$c$} (-2,-2) node(UC){$Tb$} (-4,-2) node(C){$b$};
  \path                     (-2,0) node(UX){$Tx$}  (-4,0) node(X){$x$};
  \draw[<-] (B) -- node[below]{$v$} (UC);
  \draw[<-] (B) -- node[right]{$g$} (UX);
  \draw[<-,dotted] (UC) -- node[left]{$T\hat g$} (UX);
  \draw[<-,dotted] (C) -- node[left]{$\hat g$} (X);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: