Strong Lensing of Gravitational Waves as Seen by LISA

We discuss strong gravitational lensing of gravitational waves from the merging of massive black hole binaries in the context of the LISA mission. Detection of multiple events would provide invaluable information on competing theories of gravity, evolution and formation of structures and, possibly, constraints on H-0 and other cosmological parameters. Most of the optical depth for lensing is provided by intervening massive galactic halos, for which wave optics effects are negligible. Probabilities to observe multiple events are sizable for a broad range of formation histories. For the most optimistic models, up to less than or similar to 4 multiple events with a signal to noise ratio greater than or similar to 8 are expected in a 5-year mission. Chances are significant even for conservative models with either light (less than or similar to 60%) or heavy (less than or similar to 40%) seeds. Because of lensing amplification, some intrinsically too faint signals are brought over threshold (less than or similar to 2 per year).