Double Universal Fourier Series

Abstract

In this paper we construct an integrable function of two variables for which the double Fourier-Walsh series converges both by rectangles and by spheres. Besides, we show that the coefficients of the series on the spectrum are positive and are arranged in decreasing order in all directions. Also, it is proved that after a suitable choice of signs for the Fourier coefficients of the series the spherical partial sums of the obtained series are dense in Lp[0, 1]2, p ∈ (0, 1).