@article{DE_1987_23_4_a11,
     author = {A. L. Teptin},
     title = {On the sign of the {Green} function},
     journal = {Differencialʹnye uravneni\^a},
     pages = {670--674},
     publisher = {mathdoc},
     volume = {23},
     number = {4},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_4_a11/}
}