TY  - JOUR
AU  - L. Z. Fishman
TI  - Preservation of the properties of second-order differential
JO  - Doklady Akademii Nauk
PY  - 1997
SP  - 739
EP  - 741
VL  - 352
IS  - 6
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DAN_1997_352_6_a4/
LA  - ru
ID  - DAN_1997_352_6_a4
ER  -