Relative index pairing and odd index theorem for even dimensional manifolds

We prove an analogue for even dimensional manifolds of the Atiyah–Patodi–Singer twisted index theorem for trivialized flat bundles. We show that the eta invariant appearing in this result coincides with the eta invariant by Dai and Zhang up to an integer. We also obtain the odd dimensional counterpart for manifolds with boundary of the relative index pairing by Lesch, Moscovici and Pflaum.