@article{BASS_2010_35_1_a4,
     author = {S. Todor\v{c}evi\'c},
     title = {Walks on countable ordinals and selective ultrafilters},
     journal = {Bulletin de l'Acad\'emie serbe des sciences. Classe des sciences math\'ematiques et naturelles},
     pages = {65 - 78},
     publisher = {mathdoc},
     volume = {35},
     number = {1},
     year = {2010},
     url = {http://geodesic.mathdoc.fr/item/BASS_2010_35_1_a4/}
}