\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(0, 10)(0,0)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=3}
    \thinlines
    \node[Nmarks=if,iangle=180,fangle=180](A1)(0,0){$1$}
    \drawloop[loopangle=90](A1){$a^2+b^2+(ab+ba)(a^2+b^2)^*(ab+ba)$}
    \end{picture}
  \end{center}
  \caption{Après élimination de l'état $3$.}
\end{figure}