  By the example of the Hubbard model we analytically and numerically examine
the formating and coexisting of localized electron--electron pairs (doublons)
and localized electron--hole pairs (Frenkel--type excitons) . Here we
demonstrate that at a variation of the on-site Coulomb repulsion U there occurs
a quantum transition from the doublon to the exciton regime that is conditioned
by the low-energy effective hybridization of the valence and conductivity
subbands. We calculate momentum distribution functions and electronic spectrum
functions for different U, which reveal topologically nontrivial behaviour at
the Fermi level.
