@article{700807,
     title = {Ramification systems and spaces of ultrafilters},
     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 = {1972},
     pages = {333-333},
     mrnumber = {MR0351816},
     url = {http://dml.mathdoc.fr/item/700807}
}

Negrepontis, S. Ramification systems and spaces of ultrafilters, dans General Topology and its Relations to Modern Analysis and Algebra, GDML_Books,  (1972), p. 333. http://gdmltest.u-ga.fr/item/700807/