@article{10_4153_CMB_1977_001_6,
     author = {Allegretto, W.},
     title = {A {Kneser} {Theorem} for {Higher} order {Elliptic} {Equations}},
     journal = {Canadian mathematical bulletin},
     pages = {1--8},
     year = {1977},
     volume = {20},
     number = {1},
     doi = {10.4153/CMB-1977-001-6},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1977-001-6/}
}