Let $\varphi$ denote Euler's totient function. Evaluate the series:
$$\mathcal{S}=\sum_{n=1}^{\infty} \frac{\varphi(n)}{2^n-1}$$
Solution
$$\mathcal{S}=\sum_{n=1}^{\infty} \frac{\varphi(n)}{2^n-1}$$
Solution
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