  We study the effect of correlation on the Bragg reflection in the 3D electron
gas, the 1D Luttinger liquid, and the 1D Hubbard model in an alternating
periodic potential at half-filling. In the last system, we suggest a
Luttinger-liquid-type quasi-metallic state in the crossover region from the
band insulator to the Mott insulator. We explain the appearance of this state
in terms of the incompatibility of the Bragg reflection with the concept of
Luttinger liquids.
