Uniform Distribution Theory
Journal license

Journal

Uniform Distribution Theory


Volume
& Issue

Volume 12, Issue 2


Published
on

January 10, 2017


Pages

77-90


DOI

Article

On the Closure of the Image of the Generalized Divisor Function

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Authors

Carlo Sanna Affiliation:
Department of Mathematics Universit`a degli Studi di Torino Torino, Via Carlo Alberto, 10, 10123 ITALY


Abstract

For any real number \( s \), let \( \sigma_s \) be the generalized divisor function, i.e., the arithmetic function defined by \( \sigma_s(n) := \sum_{d \mid n} d^s \), for all positive integers \( n \). We prove that for any \( r > 1 \) the topological closure of \( \sigma_{-r}(\mathbb{N}^+) \) is the union of a finite number of pairwise disjoint closed intervals \( I_1, \ldots, I_\ell \). Moreover, for \( k = 1, \ldots, \ell \), we show that the set of positive integers \( n \) such that \( \sigma_{-r}(n) \in I_k \) has a positive rational asymptotic density \( d_k \). In fact, we provide a method to give exact closed form expressions for \( I_1, \ldots, I_\ell \) and \( d_1, \ldots, d_\ell \), assuming to know \( r \) with sufficient precision. As an example, we show that for \( r = 2 \) it results \( \ell = 3 \), \( I_1 = [1, \pi^2/9] \), \( I_2 = [10/9, \pi^2/8] \), \( I_3 = [5/4, \pi^2/6] \), \( d_1 = 1/3 \), \( d_2 = 1/6 \), and \( d_3 = 1/2 \).


Keywords

Arithmetic functions, sum of divisors, topological closure, asymptotic densities.


Citation

Sanna, C. (2017). On the closure of the image of the generalized divisor function. Uniform Distribution Theory, 12(2), 77–90. https://doi.org/10.1515/udt-2017-0016

Published by: Engineering Journals

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