  The conduction electron density of states nearby a single magnetic impurity,
as measured recently by scanning tunneling microscopy (STM), is calculated. It
is shown that the Kondo effect induces a narrow Fano resonance as an intrinsic
feature in the conduction electron density of states. The line shape varies
with the distance between STM tip and impurity, in qualitative agreement with
experiments, and is sensitive to details of the band structure. For a Co
impurity the experimentally observed width and shift of the Kondo resonance are
in accordance with those obtained from a combination of band structure and
strongly correlated calculations.
