Abstract
The following general problem is studied: Given a positive integer \(c\) and two multiplicative functions \(f\) and \(g\), it is required to determine for what values of \(n\) the equality \(f(n)-g(n)=c\) holds. It is proved that, under certain constraints on the functions \(f\) and \(g\) and the solutions (in particular, under the constraint \(f(n)>g(n)\) for \(n>1\)), this equation has at most \(c^{1-\epsilon}\) solutions.
For the equation \(n-\varphi(n)=c\), it is proved that the number of solutions equals
where \(G(k)\) is the number of ways in which \(k\) can be represented as a sum of two primes.
This result is based on an assertion concerning configurations of points and straight lines.
Similar content being viewed by others
References
P. Erdős, “On the normal number of prime factors of \(p-1\) and some other related problems, concerning Euler’s \(\phi\)-function,” Quart. J. Math. (Oxford Ser.) 6, 205–213 (1935).
C. Pomerance, “Popular values of Euler’s function,” Mathematika 27 (1), 84–89.
W. D. Banks and F. Luca, Noncototients and Nonaliquots math/0409231 (2004).
E. Szemerédi and W. T. Trotter, Jr., “Extremal problems in discrete geometry,” Combinatorica 3 (3-4), 381–392 (1983).
Author information
Authors and Affiliations
Corresponding author
Additional information
Translated from Matematicheskie Zametki, 2021, Vol. 109, pp. 608-615 https://doi.org/10.4213/mzm12280.
Rights and permissions
About this article
Cite this article
Semchankau, A.S. On Differences of Multiplicative Functions and Solutions of the Equation \(n-\varphi(n)=c\). Math Notes 109, 623–629 (2021). https://doi.org/10.1134/S0001434621030329
Received:
Revised:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1134/S0001434621030329