@article{DAN_1957_117_2_a6,
     author = {I. D. Stupina},
     title = {Properties of some $\delta s$-operations},
     journal = {Doklady Akademii Nauk},
     pages = {188--190},
     publisher = {mathdoc},
     volume = {117},
     number = {2},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_117_2_a6/}
}