@article{EJDE_1998__1998__a13,
     author = {Castro,  Alfonso and Cossio,  Jorge and Neuberger,  John M.},
     title = {A minimax principle, index of the critical point, and existence of sign-changing solutions to elliptic boundary value problems},
     journal = {Electronic journal of differential equations},
     year = {1998},
     volume = {1998},
     zbl = {0901.35028},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_1998__1998__a13/}
}