@article{articlebf6c9a7a,
title   = {Shortening null geodesics in Lorentzian manifolds: applications to closed light rays},
author = {Masiello, Antonio and Piccione, Paolo},
year = {1998},
doi = {10.1016/s0926-2245(97)00020-x},
journal   = {Differential Geometry and its Applications}
}