@article{IIMI_2019_54_a5,
     author = {V. N. Ushakov and A. A. Ershov and M. V. Pershakov},
     title = {On one addition to evaluation by {L.} {S.~Pontryagin} of the geometric difference of sets in a plane},
     journal = {Izvestiya Instituta Matematiki i Informatiki Udmurtskogo Gosudarstvennogo Universiteta},
     pages = {63--73},
     year = {2019},
     volume = {54},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IIMI_2019_54_a5/}
}