@article{article06ce9218,
title   = {The existence of Liapunov functions for some non-conservative positional mechanical systems},
author = {Cesar, Mauro de Oliveira and Barone Netto, Angelo},
year = {1991},
doi = {10.1016/0022-0396(91)90140-5},
journal   = {Journal of Differential Equations}
}