Prolongement analytique des séries de Fourier sur un groupe compact

We study the Fourier series of a central function on a compact simple Lie group. We show that, if its coefficients are interpolated by an analytic function verifying a growth condition, then the sum of the series admits an analytic extension on a domain of the complexified group that in general strictly contains the domain of convergence. This result can be seen as a generalization of a theorem by Lindelöf (see [5], ch. 5).     

Absolutely convergent Fourier series and function classes

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 either to one of the Lipschitz classes Lip(α) and lip(α) for some 0<α?1, or to one of the Zygmund classes Λ_∗(1) and λ_∗(1). Our theorems generalize some of those by Boas [R.P. Boas Jr., Fourier series with positive coefficients, J. Math. Anal. Appl. 17 (1967) 463-483] and one by Németh [J. Németh, Fourier series with positive coefficients and generalized Lipschitz classes, Acta Sci. Math. (Szeged) 54 (1990) 291-304]. We also prove a localized version of a theorem by Paley [R.E.A.C. Paley, On Fourier series with positive coefficients, J. London Math. Soc. 7 (1932) 205-208] on the existence and continuity of the derivative of f.     

Ingham-Beurling type theorems with weakened gap conditions

Completing a series of works begun by Wiener [34], Paley and Wiener [28] and Ingham [9], a far-reaching generalization of Parseval"s identity was obtained by Beurling [4] for nonharmonic Fourier series whose exponents satisfy a uniform gap condition. Later this gap condition was weakened by Ullrich [33], Castro and Zuazua [5], Jaffard, Tucsnak and Zuazua [11] and then in [2] in some particular cases. In this paper we prove a general theorem which contains all previous results. Furthermore, applying a different method, we prove a variant of this theorem for nonharmonic Fourier series with vector coefficients. This result, partly motivated by control-theoretical applications, extends several earlier results obtained in [15] and [2]. Finally, applying these results we obtain an optimal simultaneous observability theorem concerning a system of vibrating strings. This revised version was published online in June 2006 with corrections to the Cover Date.     

Integration of Banach-valued functions and Haar series with Banach-valued coefficients

It is proved that for any Banach space each everywhere convergent Haar series with coefficients from this space is the Fourier–Haar series in the sense of Henstock type integral with respect to a dyadic differential basis. At the same time, the almost everywhere convergence of a Fourier–Henstock–Haar series of a Banach-space-valued function essentially depends on properties of the space.     

Ramanujan’s Master Theorem for the Hypergeometric Fourier Transform Associated with Root Systems

Ramanujan’s Master theorem states that, under suitable conditions, the Mellin transform of an alternating power series provides an interpolation formula for the coefficients of this series. Ramanujan applied this theorem to compute several definite integrals and power series, which explains why it is referred to as the “Master Theorem”. In this paper we prove an analogue of Ramanujan’s Master theorem for the hypergeometric Fourier transform associated with root systems. This theorem generalizes to arbitrary positive multiplicity functions the results previously proven by the same authors for the spherical Fourier transform on semisimple Riemannian symmetric spaces.     

On convergence of Fourier series of Besicovitch almost periodic functions

The paper deals with convergence of the Fourier series of q-Besicovitch almost periodic functions of the form $$ f(t)\sim \mathop{\sum}\limits_{m=1}^{\infty }{a_m}{{\mathrm{e}}^{{-\mathrm{i}{\uplambda_m}t}}}, $$ where {λm} is a Dirichlet sequence, that is, a strictly increasing sequence of nonnegative numbers tending to infinity. In particular, we show that, for 1 < q < ∞, the Fourier series of f(t) converges in norm to the function f(t) itself with usual order, which is analogous to the convergence in norm of the Fourier series of a function on [0, 2π]. A version of the Carleson–Hunt theorem is also investigated.     

酉羣上的富理埃分析 Ⅰ.富理埃級数的Abel求和及Dirichlet核

<正> §1.1.引言 一个变数的Fourier分析,現在已經有了丰富的成果,不少問題得到了圓滿而完整的解决,在很多人的研究中达到了很深刻的地步.当然,一个变数的Fourier分析在数学的不少其他領域起着重要的作用.可是多个变数的Fourier分析情况就不完全是如此,在有些內容上是有些不完整之处的.至于更一般的在任意紧致羣上的Fourier分析,那末,众所周知的,重要的結果只是Peter-Weyl定理,它告訴我們說:紧致羣上的連續函数可以用     

二重福里哀级数系数的性质

<正> §1.引言.设 f(x)为以2π为周期的周期函数,其福里哀展开式为下列各事是大家熟知的:设 f(x)在一个基本区间(0,2π)不有界变差的函数,则     

關於有一無限極限的一個函數的窣锇Ъ墧祵墩笖悼偤托

<正> 1.我們已經證明開於有一無限極限的一個單調函數的福里哀級數對於负指數(c,r)總和性的情形的定理,很自然地,人們還要問起:對於正指數的情形是怎麼樣?現在進行討論如下.     

On strong Nörlund summability of Fourier series

In this note, a sufficient condition for summability of Fourier series has been obtained which in conjunction with the author's Tauberian theorem [M.L. Mittal, A Tauberian theorem on strong Nörlund summability, J. Indian Math. Soc. 44 (1980) 369-377] on strong Nörlund summability gives a sufficient condition for summability [C,1,2] of a Fourier series. This generalizes results due to Prasad [G. Prasad, On strong Nörlund summability of Fourier series, Univ. Roorkee Res. J. 9 (1966-1967) 1-10] and Varshney [O.P. Varshney, Note on H₂ summability of Fourier series, Boll. Un. Mat. Ital. 16 (1961) 383-385].     

Divergence of the Fourier series by generalized Haar systems at points of continuity of a function

We obtain a connection between the Dirichlet kernels and partial Fourier sums by generalized Haar and Walsh (Price) systems. Based on this, we establish an interrelation between convergence of the Fourier series by generalized Haar and Walsh (Price) systems. For any unbounded sequence we construct a model of continuous function on a group (and even on a segment [0, 1]), whose Fourier series by generalized Haar system generated by this sequence, diverges at some point.     

Spaces of Functions of Generalized Bounded Variation. II: Questions of Uniform Convergence of Fourier Series

Tests are given for uniform convergence of Fourier series for spaces of functions of generalized bounded variation; along with the well-known tests (of Salem–Oskolkov–Young, Chanturiya, and Waterman) we suggest new tests. We show that the Waterman test for uniform convergence of Fourier series is strongest and unimprovable. We present a theorem on exact estimates for the Fourier coefficients for spaces of functions of bounded variation which contains classical results, improves several well-known results, and gives some new results.     

О скорсти сходимости рядов фурье двумерных функций ограниченной вариации

S. A. Telyakovskii [12] generalized a theorem of Bojani? [2] on the quantitative version of the Dirichlet-Jordan test, well-known in the classical Fourier analysis. Our goal is to extend Telyakovskii’s theorem from single to double Fourier series of functions in two variables that are of bounded variation in the sense of Hardy and Krause. The related theorems of Hardy [5] and Móricz [7] for such functions are corollaries of our Theorem proved in this paper.     

Splineapproximationen von beliebigem Defekt zur numerischen Lösung gewöhnlicher Differentialgleichungen. Teil III

Summary In the first part [1] a general procedure is presented to obtain polynomial spline approximations for the solutions of initial value problems for ordinary differential equations; furthermore a divergence theorem is proved there. Sufficient conditions for convergence of the method are given in the second part [2]. The remaining case which has not been considered in [1] and [2] is treated in the present paper. In this special case the procedure is equivalent to an unstable two-step method with special initial values; nevertheless, convergence can be proved. Finally,A ₀-stability of the method as well as the influence of rounding errors are investigated.

     

On the approximate form of Kluvánek's theorem

An abstract form of the classical approximate sampling theorem is proved for functions on a locally compact abelian group that are continuous, square-integrable and have integrable Fourier transforms. An additional hypothesis that the samples of the function are square-summable is needed to ensure the convergence of the sampling series. As well as establishing the representation of the function as a sampling series plus a remainder term, an asymptotic formula is obtained under mild additional restrictions on the group. In conclusion a converse to Kluvánek's theorem is established.     

On the generalization of the Salem test

The assertion that the Salem test [5] for the uniform convergence of a trigonometric Fourier series is improvable, is proved. In particular, an example of a continuous function, which does not fulfill the condition of the Salem test but satisfies the condition of the generalized Salem test [10], is constructed.Besides, the theorem which improves Golubov’s [3,4] result for continuous functions of two variables, is given.     

An embedding theorem for functions of class lip 1

We prove a theorem giving necessary and sufficient conditions for embedding the class Lip 1 in a class of functions which is defined in terms of the absolute convergence of series of Fourier coefficients with respect to the Faber-Schauder system, normalized in L₂.     

SOME REMARKS ON HOLOMORPHIC FUNCTIONS AND TAYLOR SERIES IN Cn

Some previous results on convergence of Taylor series in Cn [3] are improved by indicating outside the domain of convergence the points where the series diverges and simplifying some proofs. These results contain the Cauchy-Hadamard theorem in C. Some Cauchy integral formulas of a holomorphic function on a closed ball in Cn are constructed and the Taylor series expansion is deduced.     

关于正弦级数$\bm{L^{1}}$-收敛性研究中对单调递减条件的确切推广

本文在处理$L^1$-收敛性问题中给出了一个确切的条件和一种更直接的方式.