  In order to explore why the multi-layered cuprates have such high Tc's, we
have examined various inter-layer processes. Since the inter-layer one-electron
hopping has little effects on the band structure, we turn to the inter-layer
pair hopping. The superconductivity in a double-layer Hubbard model with and
without the inter-layer pair hopping, as studied by solving the Eliashberg
equation with the fluctuation exchange approximation, reveals that the
inter-layer pair hopping acts to increase the pairing interaction and the
self-energy simultaneously, but that the former effect supersedes the latter
and enhances the superconductivity. The inter-layer pair hopping considered
here is for off-site pairs, for which we discuss the effect of retaining SU(2)
symmetry, along with how the the sign of the pair hopping determines the
relative configuration of d-waves between the adjacent layers.
