@article{700879,
     title = {Types of ultrafilters on countable sets},
     booktitle = {General Topology and its Relations to Modern Analysis and Algebra},
     series = {GDML\_Books},
     publisher = {Academia Publishing House of the Czechoslovak Academy of Sciences},
     address = {Praha},
     year = {1967},
     pages = {142-143},
     zbl = {0206.52004},
     url = {http://dml.mathdoc.fr/item/700879}
}

Frolík, Z. Types of ultrafilters on countable sets, dans General Topology and its Relations to Modern Analysis and Algebra, GDML_Books,  (1967), pp. 142-143. http://gdmltest.u-ga.fr/item/700879/