The η-invariant of mapping tori with finite monodromies

The η-invariant of Riemannian 3-manifolds is defined by means of the spectrum of a certain elliptic operator. In this paper, we give a geometric interpretation of the deviation from the multiplicativity of the η-invariant for finite coverings. We then apply it to mapping tori with finite monodromies, and obtain a simple formula of the η-invariant for it.