# Fourier Series Tolstov Book Question

1. ### Chacabucogod

53
I'm currently reading Tolstov's Fourier series and in page 58 he talks about a criterion for the completeness of a system. I've attached the pages where he talks about it. In general, I don't get how he came up with this; specifically, why does he use those limits of $\epsilon$ and $\epsilon$/4?

-Thank you

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