  We study the localization of collective pair excitations in
weakly-interacting Bose superfluids in one-dimensional quasiperiodic lattices.
The localization diagram is first determined numerically. For intermediate
interaction and quasiperiodic amplitude we find a sharp localization
transition, with extended low-energy states and localized high-energy states.
We then develop an analytical treatment, which allows us to quantitatively map
the localization transition into that of an effective multiharmonic
quasiperiodic system.
