×

On the capacity of sets of divergence associated with the spherical partial integral operator. (English) Zbl 1035.42003

The author obtains some pointwise convergence results for the spherical summation operators \[ S_R f(x) := \int_{| \xi| \leq R} \hat f(\xi) e^{2\pi i x \cdot \xi}d\xi \] in higher dimensions \(\mathbb R^n\), \(n > 1\). The pointwise convergence of this operator for \(L^2\) functions \(f\) is still an important open problem. However, the author shows that information on the set of divergence can be obtained if more regularity is assumed on the function. For instance, if \(f\) lies in the Sobolev space \(W^{\alpha,p}\) for \(2 \leq p < 2n/(n-1)\) and \(0 < \alpha \leq n/p\) then one has pointwise convergence outside of a set of \(C_{\alpha-\varepsilon,2}\)-capacity zero for all \(\varepsilon > 0\). (This is the case if one defines \(f\) properly, as the fractional integral of its \(\alpha^{th}\) derivative). The author conjectures that this \(\varepsilon\) can be removed, and that the convergence is uniform on open sets outside of the set of divergence; the former conjecture is proven when \(\alpha > (n-1)/2\) or on any open set where \(f\) is smooth. Once \(\alpha\) is greater than or equal to \((n-1)/2\) one has a form of the Riemann localization theorem, in that one has pointwise convergence in every open set where \(f\) is smooth, but this fails for \(\alpha\) less than \((n-1)/2\). Some further improvements are obtained assuming that the set of divergence is closed. The case \(p<2\) is also discussed; the situation there is worse. For instance, the author shows that if \(1 \leq p < 2-1/n\) and \(0 \leq \alpha < ((2-p)n-1)/2p\), then the partial sums can diverge everywhere.

MSC:

42B05 Fourier series and coefficients in several variables
31B15 Potentials and capacities, extremal length and related notions in higher dimensions
Full Text: DOI

References:

[1] David R. Adams and Lars Inge Hedberg, Function spaces and potential theory, Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences], vol. 314, Springer-Verlag, Berlin, 1996. · Zbl 0545.31011
[2] A. Beurling, Ensembles exceptionnels, Acta Math. 72 (1940), pp. 1-13. · JFM 66.0449.01
[3] Anthony Carbery, The boundedness of the maximal Bochner-Riesz operator on \?\(^{4}\)(\?&sup2;), Duke Math. J. 50 (1983), no. 2, 409 – 416. · Zbl 0522.42015
[4] H. A. Slim, On co-recursive orthogonal polynomials and their application to potential scattering, J. Math. Anal. Appl. 136 (1988), no. 1, 1 – 19. · Zbl 0672.42017 · doi:10.1016/0022-247X(88)90111-4
[5] Anthony Carbery and Fernando Soria, Almost-everywhere convergence of Fourier integrals for functions in Sobolev spaces, and an \?&sup2;-localisation principle, Rev. Mat. Iberoamericana 4 (1988), no. 2, 319 – 337. · Zbl 0692.42001 · doi:10.4171/RMI/76
[6] Anthony Carbery and Fernando Soria, Pointwise Fourier inversion and localisation in \?\(^{n}\), Proceedings of the conference dedicated to Professor Miguel de Guzmán (El Escorial, 1996), 1997, pp. 847 – 858. · Zbl 0896.42007 · doi:10.1007/BF02656490
[7] A. Carbery and F. Soria, personal communication (MSRI (Berkeley, USA), 1997).
[8] Lennart Carleson, On convergence and growth of partial sums of Fourier series, Acta Math. 116 (1966), 135 – 157. · Zbl 0144.06402 · doi:10.1007/BF02392815
[9] Lennart Carleson, Selected problems on exceptional sets, Van Nostrand Mathematical Studies, No. 13, D. Van Nostrand Co., Inc., Princeton, N.J.-Toronto, Ont.-London, 1967. · Zbl 0189.10903
[10] Michael Christ, On almost everywhere convergence of Bochner-Riesz means in higher dimensions, Proc. Amer. Math. Soc. 95 (1985), no. 1, 16 – 20. · Zbl 0569.42011
[11] Charles Fefferman, A note on spherical summation multipliers, Israel J. Math. 15 (1973), 44 – 52. · Zbl 0262.42007 · doi:10.1007/BF02771772
[12] Richard A. Hunt, On the convergence of Fourier series, Orthogonal Expansions and their Continuous Analogues (Proc. Conf., Edwardsville, Ill., 1967) Southern Illinois Univ. Press, Carbondale, Ill., 1968, pp. 235 – 255.
[13] V. A. Il\(^{\prime}\)in, Localization and convergence problems for Fourier series in fundamental function systems of Laplace’s operator, Uspehi Mat. Nauk 23 (1968), no. 2 (140), 61 – 120 (Russian).
[14] Jean-Pierre Kahane and Raphaël Salem, Ensembles parfaits et séries trigonométriques, Actualités Sci. Indust., No. 1301, Hermann, Paris, 1963 (French). · Zbl 0112.29304
[15] Yūichi Kanjin, Convergence and divergence almost everywhere of spherical means for radial functions, Proc. Amer. Math. Soc. 103 (1988), no. 4, 1063 – 1069. · Zbl 0671.42016
[16] Carlos E. Kenig and Peter A. Tomas, Maximal operators defined by Fourier multipliers, Studia Math. 68 (1980), no. 1, 79 – 83. · Zbl 0442.42013
[17] Pertti Mattila, Spherical averages of Fourier transforms of measures with finite energy; dimension of intersections and distance sets, Mathematika 34 (1987), no. 2, 207 – 228. · Zbl 0645.28004 · doi:10.1112/S0025579300013462
[18] C. Meaney and E. Prestini, On convergence of some integral transforms, Miniconference on probability and analysis (Sydney, 1991) Proc. Centre Math. Appl. Austral. Nat. Univ., vol. 29, Austral. Nat. Univ., Canberra, 1992, pp. 145 – 162. · Zbl 0813.42007
[19] E. Montini, Questions Related with the Inversion of the Fourier Transform in Dimensions Greater than 1 for Functions in \({\mathcal{L}}_{\alpha}^p\), Ph.D. Thesis, Univ. of Edinburgh (U.K.), 1998.
[20] Elena Prestini, Almost everywhere convergence of the spherical partial sums for radial functions, Monatsh. Math. 105 (1988), no. 3, 207 – 216. · Zbl 0639.42010 · doi:10.1007/BF01636929
[21] F. E. Relton, Applied Bessel Functions, Blackie & Son Ltd., London, 1946. · Zbl 0136.05302
[22] R. Salem and A. Zygmund, Capacity of Sets and Fourier Series, Trans. Amer. Math. Soc. 59 (1946), pp. 23-41. · Zbl 0060.18511
[23] Per Sjölin, Two theorems on convergence of Fourier integrals and Fourier series, Approximation and function spaces (Warsaw, 1986) Banach Center Publ., vol. 22, PWN, Warsaw, 1989, pp. 413 – 426.
[24] Per Sjölin, Estimates of spherical averages of Fourier transforms and dimensions of sets, Mathematika 40 (1993), no. 2, 322 – 330. · Zbl 0789.28006 · doi:10.1112/S0025579300007087
[25] Elias M. Stein, Singular integrals and differentiability properties of functions, Princeton Mathematical Series, No. 30, Princeton University Press, Princeton, N.J., 1970. · Zbl 0207.13501
[26] Elias M. Stein, Harmonic analysis: real-variable methods, orthogonality, and oscillatory integrals, Princeton Mathematical Series, vol. 43, Princeton University Press, Princeton, NJ, 1993. With the assistance of Timothy S. Murphy; Monographs in Harmonic Analysis, III. · Zbl 0821.42001
[27] Elias M. Stein and Guido Weiss, Introduction to Fourier analysis on Euclidean spaces, Princeton University Press, Princeton, N.J., 1971. Princeton Mathematical Series, No. 32. · Zbl 0232.42007
[28] Terence Tao, The weak-type endpoint Bochner-Riesz conjecture and related topics, Indiana Univ. Math. J. 47 (1998), no. 3, 1097 – 1124. · Zbl 0926.42011 · doi:10.1512/iumj.1998.47.1544
[29] G. N. Watson, A Treatise on the Theory of Bessel Functions, Second Ed., Cambridge Univ. Press, Cambridge, 1944. · Zbl 0063.08184
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. In some cases that data have been complemented/enhanced by data from zbMATH Open. This attempts to reflect the references listed in the original paper as accurately as possible without claiming completeness or a perfect matching.