@article{10_4064_ba63_1_1,
     author = {Eleftherios Tachtsis},
     title = {On the {Existence} of {Free} {Ultrafilters} on $\omega $ and on {Russell-sets} in {ZF}},
     journal = {Bulletin of the Polish Academy of Sciences. Mathematics},
     pages = {1--10},
     publisher = {mathdoc},
     volume = {63},
     number = {1},
     year = {2015},
     doi = {10.4064/ba63-1-1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ba63-1-1/}
}