Abstract:
In this paper, we study the essential spectrum of a model lattice Hamiltonian describing a system with fluctuating number of particles ($0\le n\le2$) in the quasimomentum representation. The spectral properties are described in terms of the boundary values of a function of a complex variable, whose meaning is that of the kernel of the Schur complement
$$
H_{11}-z-H_{12}(H_{22}-z)^{-1}H_{12}^*.
$$
Keywords:Schur complement, quasimomentum representation, many-body problem, fluctuating number of particles, essential spectrum.