  For interacting Z_2 topological insulators with inversion symmetry, we
propose a simple topological invariant expressed in terms of the parity
eigenvalues of the interacting Green's function at time-reversal invariant
momenta. We derive this result from our previous formula involving the integral
over the frequency-momenta space. This formula greatly simplifies the explicit
calculation of Z_2 topological invariants in inversion symmetric insulators
with strong interactions.
