@article{ARM_1978_14_4_a5,
     author = {Krupka, Demeter},
     title = {Elementary theory of differential invariants},
     journal = {Archivum mathematicum},
     pages = {207--214},
     year = {1978},
     volume = {14},
     number = {4},
     mrnumber = {512763},
     zbl = {0428.58002},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1978_14_4_a5/}
}