  In this paper, we shall show that the limiting absorption principle for the
wave operator on the asymptotically Minkowski spacetime. This problem was
previously considered by [A. Vasy, J. Spect. Theory, 10,439-461 , (2020)].
Here, we employ a more transparent tool, the Mourre theory and removes an
additional condition which is imposed in his paper. Moreover, we also prove
that the anti-Feynman propagator defined by G\'erard and Wrochna coincides with
the outgoing resolvent.
