\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(40, 10)(-20,0)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node(A1)(-20,0){$1$}
    \node(A0)(0,0){$3$}
    \node(A2)(20,0){$1$}
    \drawloop[loopangle=90](A0){$a^2+b^2$}
    \drawedge(A1,A0){$ab+ba$}
    \drawedge(A0,A2){$ab+ba$}
    \end{picture}
  \end{center}
  \caption{Transitions passant par l'état $3$.}
\end{figure}