Almost everywhere divergence of Cesàro means of subsequences of partial sums of trigonometric Fourier series

Abstract

AbstractIn this paper, we investigate the relationship between pointwise convergence of the arithmetic means corresponding to the subsequence of partial Fourier sums $$(S_{k_j}f: j\in \mathbb {N})$$ ( S k j f : j ∈ N ) (for $$f\in L^1(\mathbb {T})$$ f ∈ L 1 ( T ) ) and the structure of the chosen subsequence of the sequence of natural numbers $$(k_j: j\in \mathbb {N})$$ ( k j : j ∈ N ) . More precisely, the problem we deal with is to provide necessary and sufficient conditions for a subsequence $$\mathcal {N}$$ N of $$\mathbb {N}$$ N that has the following property: for any subsequence $$\mathcal {N^{\prime }} = (k_j: j\in \mathbb {N})$$ N ′ = ( k j : j ∈ N ) of $$\mathcal {N}$$ N and any $$f\in L^1(\mathbb {T})$$ f ∈ L 1 ( T ) one has $$\frac{1}{N}\sum _{j=1}^N S_{k_j}f(x) \rightarrow f(x)$$ 1 N ∑ j = 1 N S k j f ( x ) → f ( x ) for a.e. $$x\in \mathbb {T}$$ x ∈ T . A direct corollary of this paper’s main theorem is that there exists a subsequence $$(k_j)$$ ( k j ) of the sequence of natural numbers and an integrable function f such that the arithmetic means of $$S_{k_j}f$$ S k j f do not converge to f almost everywhere. This is a negative answer to a question that originated in an article by Zalcwasser in 1936 Zalcwasser (Stud. Math. 6, 82–88 (1936)) for some increasing sequences $$(k_j)$$ ( k j ) of natural numbers.