Introduction to Analytic Number Theory
《Introduction to Analytic Number Theory》 · 第 160 页 · Section 8.3「Ramanujan's sum and generalizations」，Definition
Let $n$ be a fixed positive integer and consider the sum of the $n$th powers of the primitive $k$th roots of unity. This sum is known as Ramanujan's sum and is denoted by $c_k(n)$.