Absolutely convergent Fourier series and function classes. II ✩

We study the smoothness property of a function f with absolutely convergent Fourier series, and give best possible sufficient conditions in terms of its Fourier coefficients to ensure that f belongs to one of the Zygmund classes Λ∗(α) and λ∗(α) for some 0 < α 2. This paper is a natural supplement to our earlier one [F. Móricz, Absolutely convergent Fourier series and function classes, J. Math. Anal. Appl. 324 (2) (2006) 1168–1177] under the same title, and we keep its notations. © 2008 Elsevier Inc. All rights reserved