Topological stability of Majorana zero-modes in superconductor-topological insulator systems

We derive an index theorem for zero-energy Majorana fermion modes in a superconductor-topological insulator system in both two and three dimensions, which is valid for models with chiral symmetry as well as particle-hole symmetry. For more generic models without chiral symmetry, we suggest that Majorana zero-modes are classified by Z$_2$.
Comment: 8 pages, v2:discussion on class D is revised, refs added, typos corrected. v3: typos corrected, almost final version