Alexander Shchitov, On Approximation of the Continuous Functions of Two Variables by the Fourier-Haar ”Angle”, IJARM Volume 5, International Journal of Advanced Research in Mathematics (Volume 5)
https://www.scipress.com/IJARM.5.23
Abstract:
    The exact value of the upper bound of the approximation error by the construction of the type "angle" built using the Fourier-Haar partial sums is obtained on the function class of two variables in the uniform metric.
Keywords:
    Approximation, Exact Constant, Fourier-Haar ”Angle”, Fourier-Haar Sums, Fourier-Haar System, Function of Two Variables, Modulus of Continuity, Uniform Metric