Centre, commutativite et conjugaison dans un graphe de groupe
  We give characterizations of the center, of conjugated and of commuting elements in a fundamental group of a graph of group. We deduce various results : on the one hand we give a sufficient condition for the center, the centralizers, and the root structures in such a group to be in some sense trivial, and on the other hand we prove that for any group G, the conjugacy problem reduces to the same problem in a double of G along any finite family of subgroups. 
Jean-Philippe Preaux