The second four-electron singlet in the Hubbard impurity model

We consider the energy operator of four-electron systems in the Hubbard impurity model and investigate the structure of the essential spectrum and discrete spectra for the second singlet state of the system. It is shown that in the one-dimensional and two-dimensional cases the following situations exist for the essential and discrete spectrum: (a). the essential spectrum of the operator of the second singlet state of four electrons in the Hubbard impurity model consists of a union of eight segments, and the discrete spectrum of the operator consists of six eigenvalues; (b). the essential spectrum of the operator consists of a union of sixteen segments, and the discrete spectrum of the operator consists of fourteen eigenvalues; (c). the essential spectrum of the operator consists of a union of thirteen segments, and the discrete spectrum of the operator consists of nine eigenvalues; (d). the essential spectrum of the operator consists of a union of three segments, and the discrete spectrum of the operator consists of three eigenvalues. In the three-dimensional case the following situations arise: (a). the essential spectrum of an operator consists of unions of eight segments, and the discrete spectrum of the operator consists of six eigenvalues, or the essential spectrum of an operator consists of unions of three segments, and the discrete spectrum of the operator consists of three eigenvalues; (b). the essential spectrum of an operator consists of unions of eight segments, and the discrete spectrum of the operator consists of six eigenvalues; (c). the essential spectrum of an operator consists of unions of sixteen segments, and the discrete spectrum of the operator consists of fourteen eigenvalues; (d). the essential spectrum of an operator consists of unions of three segments, and the discrete spectrum of the operator consists of three eigenvalues.