Gravitational lenses in arbitrary space-times
  The precision reached by current and forthcoming strong-lensing observations
requires to accurately model various perturbations to the main deflector.
Hitherto, theoretical models have been developed to account for either
cosmological line-of-sight perturbations, or isolated secondary lenses via the
multi-plane lensing framework. This article proposes a general formalism to
describe multiple lenses within an arbitrary space-time background. The lens
equation, and the expressions of the amplification and time delays, are
rigorously derived in that framework. Our results may be applied to a wide
range of set-ups, from strong lensing in anisotropic cosmologies, to
line-of-sight perturbations beyond the tidal regime.
