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We characterize for which ultrafilters on is the ultrafilter extension of the asymptotic density on natural numbers -additive on the quotient boolean algebra or satisfies similar additive condition on . These notions were defined in [Blass A., Frankiewicz R., Plebanek G., Ryll-Nardzewski C., A Note on extensions of asymptotic density, Proc. Amer. Math. Soc. 129 (2001), no. 11, 3313–3320] under the name (null) and (*). We also present a characterization of a - and semiselective ultrafilters...
Démonstration élémentaire de la finitude de l’ensemble de type associé à une suite de densité uniforme extérieure non nulle.
In this partly expository paper we study van der Corput sets in , with a focus on connections with harmonic analysis and recurrence properties of measure preserving dynamical systems. We prove multidimensional versions of some classical results obtained for d = 1 by Kamae and M. Mendès France and by Ruzsa, establish new characterizations, introduce and discuss some modifications of van der Corput sets which correspond to various notions of recurrence, provide numerous examples and formulate some...
We introduce the concept of uniform weighted density (upper and lower) of a subset of , with respect to a given sequence of weights . This concept generalizes the classical notion of uniform density (for which the weights are all equal to 1). We also prove a theorem of comparison between two weighted densities (having different sequences of weights) and a theorem of comparison between a weighted uniform density and a weighted density in the classical sense. As a consequence, new bounds for the...
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