@article{MZM_1969_5_4_a2,
     author = {A. S. Pen and B. F. Skubenko},
     title = {Estimation from above of the period of a quadratic irrationality},
     journal = {Matemati\v{c}eskie zametki},
     pages = {413--418},
     publisher = {mathdoc},
     volume = {5},
     number = {4},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1969_5_4_a2/}
}