@article{JRAM_1974__271_151505,
     author = {G.B. Gustafson},
     title = {An integral equation for perturbed nonlinear functional differential equations, with applications to periodic solutions and nonlinear boundary value problems.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {35--62},
     publisher = {mathdoc},
     volume = {271},
     year = {1974},
     zbl = {0324.34072},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1974__271_151505/}
}