A pointwise selection principle for metric semigroup valued functions
In: Journal of Mathematical Analysis and Applications, Jg. 341 (2008-05-01), Heft 1, S. 613-625
Online
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Zugriff:
Let ∅ ≠ T ⊂ R , ( X , d , + ) be an additive commutative semigroup with metric d satisfying d ( x + z , y + z ) = d ( x , y ) for all x , y , z ∈ X , and X T the set of all functions from T into X . If n ∈ N and f , g ∈ X T , we set ν ( n , f , g , T ) = sup ∑ i = 1 n d ( f ( t i ) + g ( s i ) , g ( t i ) + f ( s i ) ) , where the supremum is taken over all numbers s 1 , … , s n , t 1 , … , t n from T such that s 1 ⩽ t 1 ⩽ s 2 ⩽ t 2 ⩽ ⋯ ⩽ s n ⩽ t n . We prove the following pointwise selection theorem: If a sequence of functions { f j } j ∈ N ⊂ X T is such that the closure in X of the set { f j ( t ) } j ∈ N is compact for each t ∈ T , and lim n → ∞ ( 1 n lim N → ∞ sup j , k ⩾ N , j ≠ k ν ( n , f j , f k , T ) ) = 0 , then it contains a subsequence which converges pointwise on T . We show by examples that this result is sharp and present two of its variants.
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A pointwise selection principle for metric semigroup valued functions
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Autor/in / Beteiligte Person: | Maniscalco, Caterina ; Chistyakov, Vyacheslav V. |
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Zeitschrift: | Journal of Mathematical Analysis and Applications, Jg. 341 (2008-05-01), Heft 1, S. 613-625 |
Veröffentlichung: | Elsevier BV, 2008 |
Medientyp: | unknown |
ISSN: | 0022-247X (print) |
DOI: | 10.1016/j.jmaa.2007.10.055 |
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