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Published online by Cambridge University Press: 17 April 2009
The following convergence criterion of Fourier series is due to M. Izumi, S. Izumi and the author:
THEOREM. Let Δ ≥ 1. If
(i) , and
(ii) as t → 0
for an a, 0 < a < 1 and for a δ, 0 < δ < π, then the Fourier series of φ(t) is convergent at the origin.
The object of this paper is to generalize the above theorem in the Hardy-Littlewood direction.