@article{DAN_1991_321_3_a0,
     author = {S. V. Belyi and \`E. R. Tsekanovskii},
     title = {Classes of operator $R$-functions and their realization by conservative systems},
     journal = {Doklady Akademii Nauk},
     pages = {441--445},
     publisher = {mathdoc},
     volume = {321},
     number = {3},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1991_321_3_a0/}
}