Almost everywhere convergence of Fej\'er means of two-dimensional triangular Walsh-Fourier series

In 1987 Harris proved (Proc. Amer. Math. Soc., 101) - among others- that for each $1\le p<2$ there exists a two-dimensional function $f\in L^p$ such that its triangular Walsh-Fourier series diverges almost everywhere. In this paper we investigate the Fej\'er (or $(C,1)$) means of the triangle two variable Walsh-Fourier series of $L^1$ functions. Namely, we prove the a.e. convergence $\sigma_n^{\bigtriangleup}f = \frac{1}{n}\sum_{k=0}^{n-1}S_{k, n-k}f\to f$ ($n\to\infty$) for each integrable two-variable function $f$

Kutatások a diadikus harmonikus analízis körében = Research in dyadic harmonic analysis

A pályázat keretében írott cikkek között számosban foglalkoztam egy és kétváltozós integrálható függvények logaritmikus közepeinek konvergenciájával. Többek között vizsgáltuk, hogy mi a legbővebb norma konvergencia tér. A kutatási időszak fő eredménye: Gát, G.: Pointwise convergence of cone-like restricted two-dimensional (C,1) means of trigonometric Fourier series, Journal of Approximation Theory, 149 (1) (2007), 74-102. Marcinkiewicz és Zygmund 1939-ben igazolta kétváltozós trigonometrikus Fourier sorok Fejér közepeivel kapcsolatban, hogy a integrálható függvények kétdimenziós Fejér közepei majdnem mindenütt a függvényhez tartanak, hacsak az közepek indexei úgy tartanak végtelenbe, hogy a hányadosuk korlátos, azaz egy egyenes köré húzott kúpban maradnak. A nevezett cikkben igazoltam, hogy, ha az egyenest helyettesítjük egy függvény görbéjével, azaz egy ''görbe köré húzott kúpban maradnak az indexek'', akkor is igaz marad a majdnem mindenütti konvergencia. Továbbá, ha a "kúp jellegű" halmaz "végtelenül bővül", akkor a tétel már nem fog teljesülni. | Among the papers written in the project I discussed the convergence of logarithmic means of one and two dimensional functions in several papers. Among others, we determinded the largest norm convergence space. The main result of the project is: Gát, G.: Pointwise convergence of cone-like restricted two-dimensional (C,1) means of trigonometric Fourier series, Journal of Approximation Theory, 149 (1) (2007), 74-102. In 1939 Marcinkiewicz and Zygmund proved with respect to the Fejér means of the trigonometric Fourier series of two variable integrable functions that if the ratio of the indices of the means remain bounded as they tend to infinity (in other words, they remain in some positive cone around of the identical function), then the Fejér means converge to the function almost everywhere. In my paper above I verified the same result for a more general case. That is, the identical function can be substituted by an "arbitrary" function. That is, the set of indices remain in a "cone-like" set ("a cone around a curve"). Moreover, if the "cone-like set" enlarges "infinitely", then the theorem fails to hold

Ces\`{a}ro means of subsequences of partial sums of trigonometric Fourier series

In 1936 Zygmunt Zalcwasser asked with respect to the trigonometric system that how "rare" can a sequence of strictly monotone increasing integers $(n_j)$ be such that the almost everywhere relation $\frac{1}{N}\sum_{j=1}^N S_{n_j}f \to f$ is fulfilled for each integrable function $f$. In this paper, we give an answer to this question. It follows from the main result that this a.e. relation holds for every integrable function $f$ and lacunary sequence $(n_j)$ of natural numbers

On almost everywhere convergence and divergence of Marcinkiewicz-like means of integrable functions with respect to the two-dimensional Walsh system

AbstractLet |n| be the lower integer part of the binary logarithm of the positive integer n and α:N2→N2. In this paper we generalize the notion of the two dimensional Marcinkiewicz means of Fourier series of two-variable integrable functions as tnαf≔1n∑k=0n−1Sα(|n|,k)f and give a kind of necessary and sufficient condition for functions in order to have the almost everywhere relation tnαf→f for all f∈L1([0,1)2) with respect to the Walsh–Paley system. The original version of the Marcinkiewicz means are defined by α(|n|,k)=(k,k) and discussed by a lot of authors. See for instance [13,8,6,3,11]