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Theorems on sets not belonging to algebras

Author(s): L. S. Grinblat
Journal: Electron. Res. Announc. Amer. Math. Soc. 10 (2004), 51-57.
MSC (2000): Primary 03E05; Secondary 54D35
Posted: May 26, 2004
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Abstract: Let $\mathcal{A}_1,\dots, \mathcal{A}_n, \mathcal{A}_{n+1}$ be a finite sequence of algebras of sets given on a set $X$, $\bigcup_{k=1}^n \mathcal{A}_k \ne \mathfrak{P}(X)$, with more than $\frac{4}{3}n $ pairwise disjoint sets not belonging to $\mathcal{A}_{n+1}$. It has been shown in the author's previous articles that in this case $\bigcup_{k=1}^{n+1} \mathcal{A}_k \ne \mathfrak{P}(X)$. Let us consider, instead of $\mathcal{A}_{n+1}$, a finite sequence of algebras $\mathcal{A}_{n+1}, \dots, \mathcal{A}_{n+l}$. It turns out that if for each natural $i \le l$ there exist no less than $\frac{4}{3}(n+l)- \frac{l}{24} $ pairwise disjoint sets not belonging to $\mathcal{A}_{n+i}$, then $\bigcup_{k=1}^{n+l} \mathcal{A}_k \ne \mathfrak{P}(X)$. Besides this result, the article contains: an essentially important theorem on a countable sequence of almost $\sigma$-algebras (the concept of almost $\sigma$-algebra was introduced by the author in 1999), a theorem on a family of algebras of arbitrary cardinality (the proof of this theorem is based on the beautiful idea of Halmos and Vaughan from their proof of the theorem on systems of distinct representatives), a new upper estimate of the function $\mathfrak{v}(n)$ that was introduced by the author in 2002, and other new results.


References:

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D. H. Fremlin, Real-valued-measurable cardinals, Set Theory of the Reals (Ramat Gan, 1991), Israel Math. Conf. Proc., vol. 6, 1993, pp. 151-304. MR 95f:03084

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M. Gitik and S. Shelah, Forcing with ideals and simple forcing notions, Israel J. Math. 68 (1989), 129-160. MR 91g:03104

[Gr1]
L. S. Grinblat, On sets not belonging to algebras of subsets, Mem. Amer. Math. Soc. 480 (1992). MR 93d:04001

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Additional Information:

L. S. Grinblat
Affiliation: Department of Mathematics, The College of Judea and Samaria, P.O.Box 3, Ariel 44837, Israel
Email: grinblat@yosh.ac.il

DOI: 10.1090/S1079-6762-04-00129-5
PII: S 1079-6762(04)00129-5
Keywords: Algebra on a set, almost $\sigma$-algebra, ultrafilter, pairwise disjoint sets
Received by editor(s): February 15, 2004
Posted: May 26, 2004
Communicated by: David Kazhdan
Copyright of article: Copyright 2004, American Mathematical Society


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