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Order of magnitude of multiple Walsh-Fourier coefficients of functions of bounded \(p\)-variation. (English) Zbl 1275.42044

Summary: For a Lebesgue integrable complex-valued function \(f\) defined over the \(n\)-dimensional torus \(\mathbb {I}^n:=[0,1)^n\) \((n\in \mathbb N)\), let \(\hat f({\mathbf k})\) denote the multiple Walsh-Fourier coefficient of \(f\), where \({\mathbf k}=(k_1,\dots,k_n)\in (\mathbb {Z}^+)^n\), \(\mathbb {Z^+}=\mathbb {N}\cup \{0\}\). The Riemann-Lebesgue lemma shows that \(\hat f({\mathbf k})=o(1)\) as \(|{\mathbf k}|\to 0\) for any \(f\in L^1(\mathbb I^n)\). However, it is known that, these Fourier coefficients can tend to zero as slowly as we wish. When \(n=1\) the definitive results are due to the author and J. R. Patadia [JIPAM, J. Inequal. Pure Appl. Math. 9, No. 2, Paper No. 44, 7 p. (2008; Zbl 1160.42012)] for functions of certain classes of functions of generalized bounded variation. The author [Acta Math. Hung. 128, No. 4, 328–343 (2010; Zbl 1240.42025)] defined the notion of bounded \(p\)-variation (\(p\geq 1\)) for a function from a rectangle \([a_1,b_1]\times\dots \times [a_n,b_n]\) to \(\mathbb {C}\) and obtained definitive results for the order of magnitude of multiple trigonometric Fourier coefficients. In this paper, such definitive results for the order of magnitude of multiple Walsh-Fourier coefficients for a function of bounded \(p\)-variation are obtained.

MSC:

42C10 Fourier series in special orthogonal functions (Legendre polynomials, Walsh functions, etc.)
42B05 Fourier series and coefficients in several variables
26B30 Absolutely continuous real functions of several variables, functions of bounded variation
26D15 Inequalities for sums, series and integrals