  We show that the local in-gap Greens function of a band insulator
$\mathbf{G}_0 (\epsilon,\mathbf{k}_{\parallel},\mathbf{r}_{\perp}=0)$, with
$\mathbf{r}_\perp$ the position perpendicular to a codimension-1 or -2
impurity, reveals the topological nature of the phase. For a topological
insulator, the eigenvalues of this Greens function attain zeros in the gap,
whereas for a trivial insulator the eigenvalues remain nonzero. This
topological classification is related to the existence of in-gap bound states
along codimension-1 and -2 impurities. Whereas codimension-1 impurities can be
viewed as 'soft edges', the result for codimension-2 impurities is nontrivial
and allows for a direct experimental measurement of the topological nature of
2d insulators.
