The Atiyah-Patodi-Singer index theorem describes the bulk-edge correspondence of symmetry protected topological insulators. The mathematical set-up for this theorem is, however, not directly related to the physical fermion system, as it imposes on the fermion fields a non-local boundary condition known as the “APS boundary condition” by hand, which is unlikely to be realized in the materials. In 2017, we showed that the same integer as the APS index can be obtained from the eta-invariant of the domain-wall Dirac operator. In this work, we invite three mathematicians to our group and prove that this correspondence is not a coincidence but generally true.