@article{SM_1968_5_2_a9,
     author = {A. S. Markus},
     title = {Letter to the {Editor}},
     journal = {Sbornik. Mathematics},
     pages = {309},
     publisher = {mathdoc},
     volume = {5},
     number = {2},
     year = {1968},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1968_5_2_a9/}
}