580 -Cardinal arithmetic (11)

March 12, 2009

4. Strongly compact cardinals and {{sf SCH}}


Definition 1 A cardinal {kappa} is strongly compact iff it is uncountable, and any {kappa}-complete filter (over any set {I}) can be extended to a {kappa}-complete ultrafilter over {I.}


The notion of strong compactness has its origin in infinitary logic, and was formulated by Tarski as a natural generalization of the compactness of first order logic. Many distinct characterizations have been found.

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Set theory seminar -Forcing axioms and inner models VII

October 24, 2008

I concluded my series of talks by showing the following theorem of Viale:

Theorem (Viale). Assume {sf CP}(kappa^+) and let Msubseteq V be an inner model where kappa is regular and such that (kappa^+)^M=kappa^+. Then {rm cf}(kappa)neomega.

This allows us to conclude, via the results shown last time, that if {sf PFA} holds in V and M computes cardinals correctly, then it also computes correctly ordinals of cofinality omega.

An elaboration of this argument is expected to show that, at least  if we strengthen the assumption of {sf PFA} to {sf MM}, then M computes correctly ordinals of cofinality omega_1

Under an additional assumption, Viale has shown this:  If {sf MM} holds in Vkappa is a strong limit cardinal, (kappa^+)^M=kappa^+, and in M we have that kappa is regular, then in V, the cofinality of kappa cannot be omega_1. The new assumption on kappa allows us to use a result of Dzamonja and Shelah, On squares, outside guessing of clubs and I_{<f} [lambda], Fund. Math. 148 (1995), 165-198, in place of the structure imposed by {sf CP}(kappa^+). It is still open if the corresponding covering statement {sf CP}(kappa^+,omega_1) follows from {sf MM}, which would eliminate the need for this the strong limit requirement.

  • Go to the intermezzo for a discussion of consistency strengths.

Set theory seminar -Forcing axioms and inner models VI

October 17, 2008

I presented a sketch of a nice proof due to Todorcevic that {sf PFA} implies the P-ideal dichotomy {sf PID}. I then introduced Viale’s covering property {sf CP} and showed that it follows from {sf PID}. Next time I will indicate how it can be used to provide a proof of part 1 of the following theorem:

Theorem (Viale). Assume Msubseteq V is an inner model.

  1. If {sf PFA} holds in V and M computes cardinals correctly, then it also computes correctly ordinals of cofinality omega.
  2. If {sf MM} holds in V, kappa is a strong limit cardinal, (kappa^+)^M=kappa^+, and in M we have that kappa is regular, then in V, the cofinality of kappa cannot be omega_1

It follows from this result and the last theorem from last time that if V is a model of {sf MM} and a forcing extension of an inner model M by a cardinal preserving forcing, then {sf ORD}^{omega_1}subset M.

In fact, the argument from last time shows that we can weaken the assumption that V is a forcing extension to the assumption that for all kappa there is a regular cardinal lambdagekappa such that  in M we have a partition S^lambda_omega=sqcup_{alpha<kappa}S_alpha where each S_alpha is stationary in V

It is possible that this assumption actually follows from {sf MM} in V. However, something is required for it: In Gitik, Neeman, Sinapova, A cardinal preserving extension making the set of points of countable V cofinality nonstationary, Archive for Mathematical Logic, vol. 46 (2007), 451-456, it is shown that (assuming large cardinals) one can find a (proper class) forcing extension of V that preserves cardinals, does not add reals, and (for some cardinal kappa) the set of points of countable V-cofinality in lambda is nonstationary for every regular lambdagekappa^+. Obviously, this situation is incompatible with {sf PFA} in V, by Viale’s result.