@article{TMF_2006_147_3_a4,
     author = {E. A. Kartashova},
     title = {A hierarchy of generalized invariants for linear partial differential},
     journal = {Teoreti\v{c}eska\^a i matemati\v{c}eska\^a fizika},
     pages = {470--478},
     year = {2006},
     volume = {147},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TMF_2006_147_3_a4/}
}