@article{article09cffff7,
title   = {Instability of equilibrium points of some Lagrangian systems},
author = {Freire Júnior, Ricardo dos Santos and Garcia, Manuel Valentim de Pera and Tal, Fábio Armando},
year = {2008},
doi = {10.1016/j.jde.2008.02.016},
journal   = {Journal of Differential Equations}
}