Studying progression on proving Wilf's conjecture

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Currently I'm studying the paper https://arxiv.org/pdf/math/0608085v1.pdf and here its written $|\{n \leq x \mid f(n) \neq 0\}|=O(x^{\frac{2}{3}})$ (where $f(n)$ is defined in the paper) can anyone please give reference to the paper where the above mentioned result is given ?