on the degree of approximation of certain continuous bivariate functions by double matrix means of a double fourier series

In this paper, we have studied the degree of approximation of certain bivariate functions by double factorable matrix means of a double fourier series. four theorems are proved using single rest bounded variation sequences, single head bounded variation sequences, double rest bounded variation sequences, and two non-negative mediate functions. these results expressed in terms of two functions of modulus type and two non-negative mediate functions, imply many particular results as shown at last section of this paper.