\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(30, 15)(0,0)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node[Nmarks=f,fangle=180](A1)(0,0){$1$}
    \node(A2)(15,0){$2$}
    \node(A3)(30,0){$3$}
    \node(A4)(30,15){$4$}
    \node(A5)(15,15){$5$}
    \node[Nmarks=i,iangle=180](A6)(0,15){$6$}
    \drawedge(A2,A1){$a$}
    \drawedge(A3,A2){$b$}
    \drawedge[ELside=r](A4,A3){$a$}
    \drawedge(A5,A4){$a$}
    \drawedge(A6,A5){$b$}
    \drawedge[ELside=r](A6,A1){$a$}
    \drawedge[ELside=r](A5,A2){$b$}
    \end{picture}
  \end{center}
  \caption{L'automate minimal du code préfixe $\{a,bba,baaba\}$.}
\end{figure}