  We calculate the modification of the local electronic structure caused by a
local impurity on the surface of a 3D Topological Insulator. We find that the
LDOS around the Dirac point of the electronic spectrum at the surface is
significantly disrupted near the impurity by the creation of low-energy
resonance state(s) -- however, this is not sufficient to (locally) destroy the
Dirac point. We also calculate the non-trivial spin textures created near the
magnetic impurities and discover anisotropic RKKY coupling between them.
