
I found this exercise in a book by Narkiewicz:
Assume that the series $f(s)=\sum_{n=1}^{\infty} a_n n^{-s}$ with real monotonic coefficients is convergent in a half-plane and satisfies there the Euler product formula
$$
f(s)=\prod_p\left(1+\sum_{k=1}^{\infty} \frac{a_{p^k}}{p^{k s}}\right).
$$
P...