Geodesic
On functions conjugate to Denjoy--Khinchin integrable functions
Izvestiya. Mathematics , Tome 5 (1971) no. 2, pp. 393-420.

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In this paper an example is constructed of a function $f(x)$ that is integrable in the sense of Denjoy–Khinchin for which the conjugate function $\bar f(x)$ is integrable in the same sense and has a positive zeroth Fourier coefficient. 
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     title = {On functions conjugate to {Denjoy--Khinchin} integrable functions},
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T. P. Lukashenko. On functions conjugate to Denjoy--Khinchin integrable functions. Izvestiya. Mathematics , Tome 5 (1971) no. 2, pp. 393-420. http://geodesic.mathdoc.fr/item/IM2_1971_5_2_a6/

[1] Khinchin A. Ya., “O protsesse integrirovaniya Denjoy”, Matem. sb., 30:4 (1918), 543–557 | Zbl

[2] Marcinkiewics J., “Sur les series de Fourier”, Fund. Math., 27 (1936), 36–69

[3] Zigmund A., Trigonometricheskie ryady, t. I, Mir, M., 1965 | MR

[4] Bari N. K., Trigonometricheskie ryady, GIFML, M., 1961 | MR