@article{AUCMP_1971_12_2_a1,
     author = {Kussov\'a, B.},
     title = {Trajectories and natural numbers},
     journal = {Acta Universitatis Carolinae. Mathematica et Physica},
     pages = {3--20},
     year = {1971},
     volume = {12},
     number = {2},
     mrnumber = {0335263},
     zbl = {0268.02052},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AUCMP_1971_12_2_a1/}
}