The topological degeneracy also appear in non-interacting fermion systems (such as p+ip superconductors) with trapped defects (such as vortices). In non-interacting fermion systems, there is only one type of topological degeneracy where number of the degenerate states is given by 2 N d / 2 / 2 , where N d is the number of the defects (such as the number of vortices). Such topological degeneracy is referred as "Majorana zero-mode" on the defects. In contrast, there are many types of topological degeneracy for interacting systems. A systematic description of topological degeneracy is given by tensor category (or monoidal category) theory.