A corollary in Katznelson's book page 21 says that:
If the Fourier series of $f\in L^1[0,2\pi]$ converges on a set $E$ of positive measure, its sum coincides with $f$ almost everywhere on $E$.
Could anyone clarify the details?
2026-03-30 00:22:51.1774830171
Pointwise convergence of Fourier series of $f\in L^1[0,2\pi]$
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