Kluyver's Formula for Ramanujan's Sum

Theorem
Let $q \in \N_{>0}$.

Let $n \in \N$.

Let $\map {c_q} n$ be Ramanujan's sum.

Let $\mu$ denote the Möbius function.

Then:


 * $\displaystyle \map {c_q} n = \sum_{d \mathop \divides \gcd \set {q, n} } d \map \mu {\frac q d}$

where $\divides$ denotes divisibility.

Proof
Let $\alpha \in \R$.

Let $e: \R \to \C$ be the mapping defined as:


 * $\map e \alpha := \map \exp {2 \pi i \alpha}$

Let $\zeta_q$ be a primitive $q$th root of unity.

Let:


 * $\displaystyle \map {\eta_q} n := \sum_{1 \mathop \le a \mathop \le q} \map e {\frac {a n} q}$

By Complex Roots of Unity in Exponential Form this is the sum of all $q$th roots of unity.

Therefore:


 * $\displaystyle \map {\eta_q} n = \sum_{d \mathop \divides q} \map {c_d} n$

By the Möbius Inversion Formula, this gives:


 * $\displaystyle \map {c_q} n = \sum_{d \mathop \divides q} \map {\eta_d} n \map \mu {\frac q d}$

Now by Sum of Roots of Unity, we have:


 * $\displaystyle \map {c_q} n = \sum_{d \mathop \divides q} d \map \mu {\frac q d}$

as required.