On the Spectrum of a Model Operator in Fock Space
  A model operator $H$ associated to a system describing four particles in interaction, without conservation of the number of particles, is considered. We describe the essential spectrum of $H$ by the spectrum of the channel operators and prove the Hunziker-van Winter-Zhislin (HWZ) theorem for the operator $H.$ We also give some variational principles for boundaries of the essential spectrum and interior eigenvalues. 
Tulkin H. Rasulov, Mukhiddin I. Muminov and Mahir Hasanov