@article{SM_1975_27_4_a7,
     author = {P. A. Kuchment and A. A. Pankov},
     title = {Letter to the {Editor}},
     journal = {Sbornik. Mathematics},
     pages = {564},
     publisher = {mathdoc},
     volume = {27},
     number = {4},
     year = {1975},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1975_27_4_a7/}
}