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2. The Rudin-Keisler ordering of P-points;Blass;Trans. Amer. Math. Soc.,1973
3. Kleene degree of ultrafilters;Blass,1984
4. Kleene degree of ultrafilters;Blass,1985
5. There may be simple Pℵ1-points and Pℵ2-points and the Rudin-Keisler ordering may be downward directed;Blass;Ann. Math. Logic,1987