@article{CMFD_2011_42_a18,
     author = {E. F. Sachkova},
     title = {Sub-Riemannian balls on the heisenberg groups: an invariant volume},
     journal = {Contemporary Mathematics. Fundamental Directions},
     pages = {199--203},
     publisher = {mathdoc},
     volume = {42},
     year = {2011},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/CMFD_2011_42_a18/}
}