@article{SMJ2_1999_25_3_a2,
     author = {El-sayed Ibrahim, Sobhy},
     title = {The {General} {Differential} {Operators} {Generated} by a {Quasi-Differential} {Expressions} with their {Interior} {Singular} {Points}},
     journal = {Serdica Mathematical Journal},
     pages = {207--240},
     publisher = {mathdoc},
     volume = {25},
     number = {3},
     year = {1999},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SMJ2_1999_25_3_a2/}
}