@article{TM_1991_196_a7,
     author = {M. D. Kovalev},
     title = {On the curvatures of tangent spheres},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {90--92},
     publisher = {mathdoc},
     volume = {196},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TM_1991_196_a7/}
}