A general theorem on Nörlund summability of Fourier series with applications
A note on a theorem of Boas
A note on a Theorem of Ermakov
A note on Fourier coefficients
A strong version of Poisson summation.
A study on degree of approximation by summability means of the Fourier-Laguerre expansion.
A survey of certain results on strong approximation by orthogonal series
This is a survey of results in a particular direction of the theory of strong approximation by orthogonal series, related mostly with author's contributions to the subject.
Absolute Nörlund summability factors of power series and Fourier series
Four theorems of Ahmad [1] on absolute Nörlund summability factors of power series and Fourier series are proved under weaker conditions.
Absolute Riesz summability of Fourier series
Almost everywhere convergence of Marcinkiewicz means of Fourier series on the group of 2-adic integers
We prove the almost everywhere convergence of the Marcinkiewicz means of integrable functions σₙf → f for every f ∈ L¹(I²), where I is the group of 2-adic integers.
An estimate of the rate convergence of Cesaro means of Fourier Stieltjes series and its conjugate series
An estimate of the rate of convergence for Fourier series of functions of bounded variation.
An estimate of the rate of convergence of the triangular matrix means of the Fourier series of functions of bounded variation.
Approximation of functions from by general linear operators of their Fourier series
We show the general and precise conditions on the functions and modulus of continuity as well as on the entries of matrices generating the summability means and give the rates of approximation of functions from the generalized integral Lipschitz classes by double matrix means of their Fourier series. Consequently, we give some results on norm approximation. Thus we essentially extend and improve our earlier results [Acta Comment. Univ. Tartu. Math. 13 (2009), 11-24] and the result of S. Lal [Appl....
Approximation of functions from by linear operators of conjugate Fourier series
We show the results corresponding to some theorems of S. Lal and H. K. Nigam [Int. J. Math. Math. Sci. 27 (2001), 555-563] on the norm and pointwise approximation of conjugate functions and to the results of the authors [Acta Comment. Univ. Tartu. Math. 13 (2009), 11-24] also on such approximations.
Approximations- und Eindeutigkeitssatz für Exponentialsysteme mit komplexen Exponenten.
Axiomatic theory of divergent series and cohomological equations
A general theory of summation of divergent series based on the Hardy-Kolmogorov axioms is developed. A class of functional series is investigated by means of ergodic theory. The results are formulated in terms of solvability of some cohomological equations, all solutions to which are nonmeasurable. In particular, this realizes a construction of a nonmeasurable function as first conjectured by Kolmogorov.