@article{ZNSL_1973_36_a3,
     author = {A. A. Ivanov},
     title = {Extension structures},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {50--125},
     publisher = {mathdoc},
     volume = {36},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1973_36_a3/}
}