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Metric discrepancy theory, functions of bounded variation and GCD sumsKeywords: Function of bounded variation , discrepancy , gcd sum. Abstract: Let $f(x)$ be a $1$-periodic function of bounded variation having mean zero, and let $(n_k)_{k \geq 1}$ be an increasing sequence of positive integers. Then a result of Baker implies the upper bound $\left| \sum_{k=1}^N f(n_k x) \right| = \mathcal{O} ( \sqrt{N} (\log N)^{3/2+ \ve} )$ for almost all $x \in (0,1)$ in the sense of the Lebesgue measure. We show that the asymptotic order of $\left| \sum_{k=1}^N f(n_k x) \right|$ is closely connected with certain number-theoretic properties of the sequence $(n_k)_{k \geq 1}$, namely a certain function involving the greatest common divisor function. More exactly, we give an upper bound for the asymptotic order of $\left| \sum_{k=1}^N f(n_k x) \right|$ in terms of the function $$h_N (n_1, \dots,n_N) = \sum_{1 \leq k_1, k_2 \leq N} \frac{\gcd(n_{k_1},n_{k_2})}{\max(n_{k_1},n_{k_2})}.$$
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