@article{702300,
     title = {The periodic boundary value problem for some second order ordinary differential equations},
     booktitle = {Equadiff 5},
     series = {GDML\_Books},
     publisher = {BSB B.G. Teubner Verlagsgesellschaft},
     address = {Leipzig},
     year = {1982},
     pages = {256-259},
     zbl = {0535.34029},
     url = {http://dml.mathdoc.fr/item/702300}
}

Mawhin, Jean. The periodic boundary value problem for some second order ordinary differential equations, dans Equadiff 5, GDML_Books,  (1982), pp. 256-259. http://gdmltest.u-ga.fr/item/702300/