Abstract. The theory of supercharacters, recently developed by DiaconisIsaacs and André, can be used to derive the fundamental algebraic properties of Ramanujan sums. This machinery frequently yields one-line proofs of difficult identities and provides many novel formulas. In addition to exhibiting a new application of supercharacter theory, this article also serves as a blueprint for future work since some of the abstract results we develop are applicable in much greater generality.
IntroductionOur primary aim in this note is to demonstrate that most of the fundamental algebraic properties of Ramanujan sums can be deduced using the theory of supercharacters, recently developed by Diaconis-Isaacs and André. In fact, the machinery of supercharacter theory frequently yields one-line proofs of many difficult identities and provides an array of new tools which can be used to derive various novel formulas. Our approach is entirely systematic, relying on a flexible and general framework. Indeed, we hope to convince the reader that supercharacter theory provides a natural framework for the study of Ramanujan sums. In addition to exhibiting a novel application of supercharacter theory, this article also serves as a blueprint for future work since some of the abstract results which we develop are applicable in much greater generality (see [10]).
Ramanujan sums.In what follows, we let e(x) = exp(2πix), so that the function e(x) is periodic with period 1. For integers n, x with n ≥ 1, the expression