On Fourier coefficients with respect to the Walsh double system

In the present paper we will consider the behavior of Fourier coefficients with respect to the Walsh double system after modification of functions. We prove that for any function $f(x,y)\in L^p[0,1]^2$ one can find a function $g(x,y)\in L^p[0,1]^2$ coinciding with $f(x,y)$ except a set of small measure such that the non-zero coefficients of $g(x,y)$ are monotonically decreasing over all rays in absolute value.