Ramanujan expansions of arithmetic functions of several variables L´aszl´oT´oth Department of Mathematics, University of P´ecs Ifj´us´ag ´utja 6, 7624 P´ecs, Hungary E-mail:
[email protected] The Ramanujan Journal 47 (2018), Issue 3, 589–603 Abstract We generalize certain recent results of Ushiroya concerning Ramanujan expansions of arithmetic functions of two variables. We also show that some properties on expansions of arithmetic functions of one and several variables using classical and unitary Ramanujan sums, respectively, run parallel. 2010 Mathematics Subject Classification: 11A25, 11N37 Key Words and Phrases: Ramanujan expansion of arithmetic functions, arithmetic function of several variables, multiplicative function, unitary divisor, unitary Ramanujan sum 1 Introduction Basic notations, used throughout the paper are included in Section 2. Further notations are explained by their first appearance. Refining results of Wintner [25], Delange [3] proved the following general theorem concerning Ramanujan expansions of arithmetic functions. Theorem 1. Let f : N Ñ C be an arithmetic function. Assume that 8 ωpnq |pµ ˚ fqpnq| arXiv:1704.02881v3 [math.NT] 12 Nov 2018 2 ă8. (1) n n“1 ÿ Then for every n P N we have the absolutely convergent Ramanujan expansion 8 fpnq“ aqcqpnq, (2) q“1 ÿ where the coefficients aq are given by 8 pµ ˚ fqpmqq a “ pq P Nq. q mq m“1 ÿ 1 Delange also pointed out how this result can be formulated for multiplicative functions f. By Wintner’s theorem ([25, Part I], also see, e.g., Postnikov [11, Ch. 3], Schwarz and Spilker [12, Ch. II]), condition (1) ensures that the mean value 1 Mpfq“ lim fpnq xÑ8 x nďx ÿ exists and a1 “ Mpfq.