## Functiones et Approximatio Commentarii Mathematici

### Quasi''-norm of an arithmetical convolution operator and the order of the Riemann zeta function

Titus Hilberdink

#### Abstract

In this paper we study Dirichlet convolution with a given arithmetical function $f$ as a~linear mapping $\varphi_f$ that sends a sequence $(a_n)$ to $(b_n)$ where $b_n=\sum_{d|n} f(d)a_{n/d}$. We investigate when this is a bounded operator on $l^2$ and find the operator norm. Of particular interest is the case $f(n)=n^{-\alpha}$ for its connection to the Riemann zeta function on the line $\Re s =\alpha$. For $\alpha>1$, $\varphi_f$ is bounded with $\|\varphi_f\|=\zeta(\alpha)$. For the unbounded case, we show that $\varphi_f:\mathcal{M}^2\to\mathcal{M}^2$ where $\mathcal{M}^2$ is the subset of $l^2$ of multiplicative sequences, for many $f\in\mathcal{M}^2$. Consequently, we study the quasi'-norm $\sup_{\substack{\|a\|=T\\ a\in\mathcal{M}^2} } \frac{\|\varphi_f a\|}{\|a\|}$ for large $T$, which measures the size' of $\varphi_f$ on $\mathcal{M}^2$. For the $f(n)=n^{-\alpha}$ case, we show this quasi-norm has a striking resemblance to the conjectured maximal order of $|\zeta (\alpha+iT)|$ for $\alpha>\frac{1}{2}$.

#### Article information

Source
Funct. Approx. Comment. Math., Volume 49, Number 2 (2013), 201-220.

Dates
First available in Project Euclid: 20 December 2013

https://projecteuclid.org/euclid.facm/1387572226

Digital Object Identifier
doi:10.7169/facm/2013.49.2.1

Mathematical Reviews number (MathSciNet)
MR3161490

Zentralblatt MATH identifier
1283.11137

Subjects
Primary: 11N37: Asymptotic results on arithmetic functions
Secondary: 11M06: $\zeta (s)$ and $L(s, \chi)$ 11A41: Primes

#### Citation

Hilberdink, Titus. Quasi''-norm of an arithmetical convolution operator and the order of the Riemann zeta function. Funct. Approx. Comment. Math. 49 (2013), no. 2, 201--220. doi:10.7169/facm/2013.49.2.1. https://projecteuclid.org/euclid.facm/1387572226

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