Abstract.
A submeasure μ defined on the subsets of \({\mathbb{N}}\) is nonatomic if for every ℓ ≥ 1 there exists a partition of \({\mathbb{N}}\) into a finite number of parts on which μ is bounded from above by 1/ℓ. In this paper we answer several natural questions concerning nonatomic submeasures d F that are determined (like the standard density) by a family F of finite subsets of \({\mathbb{N}}\). We first show that if the number of n-element sets in F grows at most exponentially with n, then d F is nonatomic; but if this growth condition fails, then d F need not be nonatomic in general. We next prove that, for a nonatomic submeasure d F , the minimal number of sets in a 1/ℓ-small partition of \({\mathbb{N}}\) can grow arbitrarily fast with ℓ. We also give a simple example of a nonatomic submeasure that is not equivalent to a submeasure of type d F .
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Received: 20 January 2008
The second author acknowledges a generous support of the Foundation for Polish Science.
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Drewnowski, L., Łuczak, T. On nonatomic submeasures on \({\mathbb{N}}\) . Arch. Math. 91, 76–85 (2008). https://doi.org/10.1007/s00013-008-2721-x
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DOI: https://doi.org/10.1007/s00013-008-2721-x