@article{articlefaa92f41,
title   = {A timelike extension of Fermat's principle in general relativity and applications},
author = {Giannoni, Fabio and Masiello, Antonio and Piccione, Paolo},
year = {1998},
doi = {10.1007/s005260050091},
journal   = {Calculus of Variations and Partial Differential Equations}
}