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On a number theoretic inequality of Ramanujan. (English) Zbl 07923067

in this short but interesting article, the authors prove that, for all integers \(n \geqslant 2\), we have \[ \tau(n) \leqslant \left( 1+\frac{\log n}{\log \gamma(n)}\right)^{\omega(n)} \] provided that, if \(n = p_1^{\alpha_1} \dotsb p_r^{\alpha_r}\), then \(p_1 < \dotsb < p_r\) and \(\alpha_1 \leqslant \dotsb \leqslant\alpha_r\). Here, \(\tau(n)\), \(\gamma(n)\) and \(\omega(n)\) are respectively the number of divisors, the squarefree kernel and the number of distinct prime divisors of \(n\). This improves on an old result of Ramanujan [S. Ramanujan, Proc. Lond. Math. Soc. (2) 14, 347–409 (1915; JFM 45.1248.01)]. The proof rests on Chebyshev’s inequality dealing with the product of arithmetic means, which can be found for instance in [D. S. Mitrinović, Analytic inequalities. In cooperation with P. M. Vasić. Berlin: Springer-Verlag (1970; Zbl 0199.38101)], Theorem 1 p. 36.

MSC:

11N37 Asymptotic results on arithmetic functions

References:

[1] B. C. Berndt, Ramanujan’s Notebooks, Part IV, Springer, New York, 1994. · Zbl 0785.11001
[2] J.-M. De Koninck and P. Letendre, New upper bounds for the number of divisors function, Colloq. Math., 162(2020), 23-52. · Zbl 1460.11115
[3] J.-M. De Koninck and F. Luca, Analytic Number Theory: Exploring the Anatomy of Integers, Amer. Math. Soc., Providence, R.I., 2012. · Zbl 1247.11001
[4] G. H. Hardy, J. E. Littlewood and G. Pólya, Inequalities, Camb. Univ. Press, 1952. · Zbl 0047.05302
[5] P. Letendre, A hybrid inequality for the number of divisors of an integer, Ann. Univ. Sci. Budapest. Sect. Comput., 52(2021), 243-254. · Zbl 1488.11150
[6] D. S. Mitrinović, Analytic Inequalities, Springer, New York, 1970. · Zbl 0199.38101
[7] S. Ramanujan, Highly composite numbers, Proc. London Math. Soc., 14(1915), 347-409. · JFM 45.0286.02
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