  It has been understood that short range interactions can reduce the
classification of topological superconductors in all dimensions. In this paper
we demonstrate by explicit calculations that when the topological phase
transition between two distinct phases in the noninteracting limit is gapped
out by interaction, the bulk fermion Green's function $G(i\omega)$ at the
"transition" approaches zero as $G(i\omega) \sim \omega$ at certain momentum
$\vec{k}$ in the Brillouin zone.
