\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(20, 32)(0,-3)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node[Nmarks=i,iangle=180](A1)(0,20){$1$}
    \node[Nmarks=f,fangle=45](A2)(20,20){$2$}
    \node(A3)(0,0){$3$}
    \node[Nmarks=f,fangle=-45](A4)(20,0){$4$}
    \drawloop[loopangle=90](A1){$b$}
    \drawedge(A1,A2){$a$}
    \drawedge(A2,A3){$b$}
    \drawedge(A3,A1){$b$}
    \drawedge[curvedepth=3](A2,A4){$a$}
    \drawedge[curvedepth=3](A4,A2){$a$}
    \drawedge[curvedepth=3](A3,A4){$a$}
    \drawedge[curvedepth=3](A4,A3){$b$}
    \end{picture}
  \end{center}
  \caption{L'automate $\cal A$.}
\end{figure}