  Unconventional features in superconductivity are revealed by responses to
impurity scattering. We study non-magnetic impurity effects in a
three-dimensional topological superconductor, focusing on an effective model
(massive Dirac Hamiltonian with s-wave on-site pairing) of Copper-doped
bismuth-selenium compounds. Using a self-consistent T-matrix approach for
impurity scattering, we examine in-gap states in density of states. We find
that the results are well characterized by a single material variable, which
measures relativistic effects in the Dirac Hamiltonian. In non-relativistic
regime, an odd-parity superconducting state is fragile against non-magnetic
impurities. We show that this behavior is caused by a p-wave character involved
in the topological superconducting state. In contrast, we show that in
relativistic regime the superconductivity is robust against non-magnetic
impurities, owing to an s-wave character. To summarize, the system has two
aspects, p- and s-wave features, depending on the weight of relativistic
effects.
