@article{702432,
     title = {Multiplicity results for nonlinear elliptic equations},
     booktitle = {Nonlinear Analysis, Function Spaces and Applications},
     series = {GDML\_Books},
     publisher = {BSB B. G. Teubner Verlagsgesellschaft},
     address = {Leipzig},
     year = {1986},
     pages = {109-138},
     mrnumber = {MR921246},
     zbl = {0633.35027},
     url = {http://dml.mathdoc.fr/item/702432}
}

Ruf, Bernhard. Multiplicity results for nonlinear elliptic equations, dans Nonlinear Analysis, Function Spaces and Applications, GDML_Books,  (1986), pp. 109-138. http://gdmltest.u-ga.fr/item/702432/