  In 3D and 2D electronic systems the singular contribution to the static
permittivity $\epsilon$ (Kohn singularity) is a small correction to the regular
part of $\epsilon$ but it results in the leading term in asymptotic behavior of
the screened potential (Friedel oscillations). In the present letter we show
that for nanotubes quite different results are valid: $\epsilon$ becomes
infinitely large at the singular point and the Friedel oscillations do not play
the dominant role in the screening at the large distances. Moreover, the zero
and highest cylindrical harmonics of the effective potential are screened by
quite different mechanisms.
