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Applications of infinitary combinatorics 2


Academic year: 2021

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Applications of infinitary combinatorics 2


Filters and ultrafilters.

Zad. 1 Show that every filter can be extended to an ultrafilter.

Zad. 2 We say that a family A generates a free filter if its closure under supersets and intersections is a free filter. Show that this is equivalent to the assertion that A has strong finite intersection property (sfip), i.e. every intersection of finitely many elements of A is infinite.

Zad. 3 Show that there are 2c many ultrafilters on ω. Hint: consider an independent family (Aα)α<c of subsets of ω. For f : c → {0, 1} show that there is an ultrafilter Ff such that Aα ∈ Ff if and only if f (α) = 1.

Zad. 4 Fix n ∈ ω and consider a family ∅ /∈ C ⊆ P(ω) satisfying the condition (?)n:

“for every partition of ω in less than n + 1 many sets the family C contains exactly one element of the partition”. For which n the condition (?)n is equivalent to “C is an ultrafilter”? Note: we do not assume here that C is a filter!

Zad. 5 If F is a filter on ω, then P is a pseudo-intersection of F if P is infinite and P ⊆ F for every F ∈ F .

• Show that every free filter generated by countably many elements does have a pseudo-intersection.

• Show that the density filter does not have a pseudo-intersection.

Zad. 6 A filter F is a P-filter if for every family {Fn: n < ω} ⊆ F there is A ∈ F such that A ⊆ Fn for every n < ω.

• Show that under Continuum Hypothesis there is a P-ultrafiler.

• Show that the density filter, i.e.

{A ⊆ ω : limA ∩ {0, . . . , n − 1}

n = 1}

is a P-filter.

• Show that the density filter cannot be extended to a P-ultrafilter.


Zad. 7 (*) Is P(ω)/Fin isomorphic to P(ω1)/Fin?




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