@article{GDZPPN002059614, title = {Hill Equations with Coexisting Periodic Solutions, II.}, journal = {Commentarii mathematici Helvetici}, volume = {44}, year = {1969}, pages = {381-384}, zbl = {0186.41003}, url = {http://dml.mathdoc.fr/item/GDZPPN002059614} }
Guggenheimer, H. Hill Equations with Coexisting Periodic Solutions, II.. Commentarii mathematici Helvetici, Tome 44 (1969) pp. 381-384. http://gdmltest.u-ga.fr/item/GDZPPN002059614/