\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(25, 35)(0,0)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=3}
    \thinlines
    \node[Nmarks=if,iangle=180,fangle=180](A1)(0,25){$1$}
    \node(A3)(25,12.5){$3$}
    \node(A4)(0,0){$4$}
    \drawloop[loopangle=90](A1){$a^2$}
    \drawloop[loopangle=0](A3){$b^2$}
    \gasset{curvedepth=3}
    \drawedge(A1,A3){$ab$}
    \drawedge(A3,A1){$ba$}
    \drawedge(A3,A4){$a$}
    \drawedge(A4,A3){$a$}
    \drawedge(A4,A1){$b$}
    \drawedge(A1,A4){$b$}
    \end{picture}
  \end{center}
  \caption{Après élimination de l'état $2$.}
\end{figure}