@article{SM_1970_11_2_a5,
     author = {S. P. Geisberg},
     title = {On the properties of the normal mapping generated by the equations $rt-s^2=-f^2(x,y)$},
     journal = {Sbornik. Mathematics},
     pages = {201--208},
     publisher = {mathdoc},
     volume = {11},
     number = {2},
     year = {1970},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_11_2_a5/}
}