\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(20, 16)(0,-1)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node[Nmarks=i,iangle=180](A1)(0,15){$1$}
    \node[Nmarks=f,fangle=90](A2)(20,15){$2$}
    \node[Nmarks=f,fangle=0](A3)(10,0){$3$}
    \drawloop[loopangle=0](A2){$b$}
    \drawedge(A1,A2){$a$}
    \drawedge(A3,A1){$a$}
    \drawedge(A2,A3){$b$}
    \end{picture}
  \end{center}
  \caption{L'automate $\cal A'$.}
\end{figure}