@article{DE_1974_10_10_a13,
     author = {N. T. Pham},
     title = {Natural differential operators on manifolds, and boundary value problems that are elliptic in a subspace},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1872--1884},
     publisher = {mathdoc},
     volume = {10},
     number = {10},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1974_10_10_a13/}
}