  Based on the two-dimensional lattice fermion model, we discuss transitions
between different pairing states. Each phase is labeled by an integer which is
a topological invariant and characterized by vortices of the Bloch
wavefunction. The transitions between phases with different integers obey a
selection rule. Basic properties of the edge states are revealed. They reflect
the topological character of the bulk. Transitions driven by randomness are
also discussed numerically.
