@article{EJDE_2006__2006__a69,
     author = {Zhao,  Chenglong and Yuan,  Yanyan and Liu,  Yansheng},
     title = {A necessary and sufficient condition for the existence of positive solutions to singular boundary-value problems of higher order differential equations},
     journal = {Electronic journal of differential equations},
     year = {2006},
     volume = {2006},
     zbl = {1120.34011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2006__2006__a69/}
}