Energy spectrum of the equal spin-spin interactions Hamiltonian
  The energy spectrum of eigenvalues of the equal spin-spin interactions (ESSI) Hamiltonian has been found. The obtained spectrum is free from limitations imposed on number of spins and parameters of the ESSI Hamiltonian. This model can be used for consideration of spin dynamics of mesoscopic systems and molecules with large number of nuclei spins. 
A.R.Kessel and R.R.Nigmatullin and N.M.Yakovleva