@article{10_4064_cm111_2_5,
     author = {Saharon Shelah},
     title = {The spectrum of characters of ultrafilters on 
$\omega$},
     journal = {Colloquium Mathematicum},
     pages = {213--220},
     publisher = {mathdoc},
     volume = {111},
     number = {2},
     year = {2008},
     doi = {10.4064/cm111-2-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm111-2-5/}
}