@article{CMUC_1981__22_3_a0,
     author = {Bukovsk\'y, Lev and Butkovi\v{c}ov\'a, Eva},
     title = {Ultrafilter with $\aleph_0$ predecessors in {Rudin-Frol{\'\i}k} order},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {429--447},
     publisher = {mathdoc},
     volume = {22},
     number = {3},
     year = {1981},
     mrnumber = {633575},
     zbl = {0495.54003},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1981__22_3_a0/}
}