@article{articled26be163,
title   = {On the spectral flow in Lorentzian manifolds},
author = {Masiello, Antonio and Piccione, Paolo},
year = {2003},
doi = {10.1007/s10231-002-0057-x},
journal   = {Annali di Matematica Pura ed Applicata, Ser. 4}
}