On the Study of the Substructure of N-body Systems
  Statistical properties of nonlinear systems revealed in an unexpected way already in Fermi-Pasta-Ulam paper, are responsible for the appearance of substructures. We represent a method within S-tree diagram formalism for investigation of the substructures of N-body gravitating systems in a defined time interval. This approach enables one to consider the link between substructures of the system and its relative instability and is efficient for the study of the substructures in clusters of galaxies. 
K.M.Bekarian, A.A.Melkonian (Yerevan Physics Institute and Garni Space   Astronomy Institute, Armenia)