@article{JRAM_1987__374_152892,
     author = {Howard A. Levine and Richard A. Smith},
     title = {A potential well theory for the wave equation with a nonlinear boundary condition.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {1--23},
     publisher = {mathdoc},
     volume = {374},
     year = {1987},
     zbl = {0598.35061},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1987__374_152892/}
}