@article{700180,
     title = {Critical point theory and nonlinear differential equations},
     booktitle = {Equadiff 6},
     series = {GDML\_Books},
     publisher = {J. E. Purkyn\v e University, Department of Mathematics},
     address = {Brno},
     year = {1986},
     pages = {49-58},
     mrnumber = {MR0877106},
     zbl = {0617.58015},
     url = {http://dml.mathdoc.fr/item/700180}
}

Mawhin, Jean. Critical point theory and nonlinear differential equations, dans Equadiff 6, GDML_Books,  (1986), pp. 49-58. http://gdmltest.u-ga.fr/item/700180/