The anticommutator Green’s functions and the energy spectra of fullerene С_36 and endohedral fullerene La@C_36, with both of them belonging to the D _6 h symmetry group, are obtained analytically within the Hubbard model in the mean-field approximation. Using group-theory methods, their energy states are classified and allowed transitions in the energy spectra of the С_36 and La@C_36 molecules, both belonging to the D _6 h symmetry group, are determined.