A strengthened form of a theorem of Fejér
Compositio Mathematica, Tome 21 (1969) no. 3, pp. 262-270. 
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     title = {A strengthened form of a theorem of {Fej\'er}},
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     volume = {21},
     number = {3},
     year = {1969},
     mrnumber = {256062},
     zbl = {0177.31802},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1969__21_3_262_0/}
}
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Siddiqi, Jamil A. A strengthened form of a theorem of Fejér. Compositio Mathematica, Tome 21 (1969) no. 3, pp. 262-270. http://archive.numdam.org/item/CM_1969__21_3_262_0/

L. Fejér [1 ] Ûber die Bestimmung des Sprunges einer Funktionen aus ihrer Fourierreihe, J. Reine Angew. Math, 142 (1913), 165-168. | EuDML | JFM

J.A. Siddiqi [2] Coefficient properties of certain Fourier series, Math. Ann., 181 (1969), 242-254. | EuDML | MR | Zbl

N. Wiener [3] The quadratic variation of a function and its Fourier coefficients, J. Math. and Phys. 3 (1924), 72-94. | JFM

A. Zygmund [4] Trigonometric Series vol. 1, Cambridge, 1959. | Zbl