@article{article01692e30,
title   = {A variational theory for light rays in stably causal Lorentzian manifolds: regularity and multiplicity results},
author = {Giannoni, Fabio and Masiello, Antonio and Piccione, Paolo},
year = {1997},
doi = {10.1007/s002200050141},
journal   = {Communications in Mathematical Physics}
}