@article{article33f59f7f,
title   = {A Morse theory for massive particles and photon in general relativity},
author = {Giannoni, Fabio and Masiello, Antonio and Piccione, Paolo},
year = {2000},
doi = {10.1016/s0393-0440(99)00045-5},
journal   = {Journal of Geometry and Physics}
}