@article{1102867208, author = {Allegretto, W.}, title = {Nonoscillation theory of elliptic equations of order $2n$.}, journal = {Pacific J. Math.}, volume = {67}, number = {2}, year = {1976}, pages = { 1-16}, language = {en}, url = {http://dml.mathdoc.fr/item/1102867208} }
Allegretto, W. Nonoscillation theory of elliptic equations of order $2n$.. Pacific J. Math., Tome 67 (1976) no. 2, pp. 1-16. http://gdmltest.u-ga.fr/item/1102867208/