@article{JRAM_1990__410_153257,
     author = {Charles A. Stuart and T. K\"upper},
     title = {Gap-bifurcation for nonlinear perturbations of {Hill's} equation.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {23--52},
     publisher = {mathdoc},
     volume = {410},
     year = {1990},
     zbl = {0704.34012},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1990__410_153257/}
}