On the essential and discrete spectrum of a model operator related to   three-particle discrete Schr\"odinger operators
  A model operator $H$ corresponding to a three-particle discrete Schr\"odinger operator on a lattice $\Z^3$ is studied. The essential spectrum is described via the spectrum of two Friedrichs models with parameters $h_\alpha(p),$ $\alpha=1,2,$ $p \in \T^3=(-\pi,\pi]^3.$ The following results are proven:   1) The operator $H$ has a finite number of eigenvalues lying below the bottom of the essential spectrum in any of the following cases: (i) both operators $h_\alpha(0), \alpha=1,2,$ have a zero eigenvalue; (ii) either $h_1(0)$ or $h_2(0)$ has a zero eigenvalue.   2) The operator $H$ has infinitely many eigenvalues lying below the bottom and accumulating at the bottom of the essential spectrum, if both operators $h_\alpha(0),\alpha=1,2,$ have a zero energy resonance. 
Sergio Albeverio, Saidakhmat N. Lakaev, Ramiza Kh. Djumanova