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Precipitous ideals

  • T. Jech (a1), M. Magidor (a2), W. Mitchell (a3) and K. Prikry (a4)

The properties of small cardinals such as ℵ1 tend to be much more complex than those of large cardinals, so that properties of ℵ1 may often be better understood by viewing them as large cardinal properties. In this paper we show that the existence of a precipitous ideal on ℵ1 is essentially the same as measurability.

If I is an ideal on P(κ) then R(I) is the notion of forcing whose conditions are sets xP(κ)/I, with xx′ if xx′. Thus a set D R(I)-generic over the ground model V is an ultrafilter on P(κ) ⋂ V extending the filter dual to I. The ideal I is said to be precipitous if κ ⊨R(I)(Vκ/D is wellfounded).

One example of a precipitous ideal is the ideal dual to a κ-complete ultrafilter U on κ. This example is trivial since the generic ultrafilter D is equal to U and is already in the ground model. A generic set may be viewed as one that can be worked with in the ground model even though it is not actually in the ground model, so we might expect that cardinals such as ℵ1 that cannot be measurable still might have precipitous ideals, and such ideals might correspond closely to measures.

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[1]Galvin F., Jech T. and Magidor M., An ideal game, this Journal, vol. 43 (1978), pp. 285292.
[2]Jech T. and Prikry K., Ideals over uncountable sets: application of almost disjoint functions and generic ultrapowers, Memoirs of the American Mathematical Society, vol. 18 (2) (1979).
[3]Jech T. and Prikry K., Bulletin of the American Mathematical Society (to appear).
[4]Kunen K., Some applicationso of iterated ultrapowers in set theory, Annals of Mathematical Logic, vol. 1 (1970), pp. 179227.
[5]Mitchell W., The core model for sequences of ultrafilters (in preparation).
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The Journal of Symbolic Logic
  • ISSN: 0022-4812
  • EISSN: 1943-5886
  • URL: /core/journals/journal-of-symbolic-logic
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