  Substrate and nanotube polarization are shown to change qualitatively a
nanotube bandstructure. The effect is studied in a linear approximation in an
external potential which causes the changes. A work function difference between
the nanotube and gold surface is estimated to be large enough to break the band
symmetry and lift a degeneracy of a lowest but one subband of a metallic
nanotube. This subband splitting for [10,10] nanotube is about 50 meV in
absence of other external potential.
