On obtaining close estimates in the approximation of functions of many variables by sums of functions of a fewer number of variables

We present a simple method for finding the values of the best approximation of a function of n variables of a given class by means of sums of two functions of a fewer number of variables; we establish close upper and lower bounds for the value of the best approximation to the functionf(x₁, ..., x_n), having the mixed derivativef x₁ ... x_n, by means of sums of a function of n–1 variables.Translated from Matematicheskie Zametki, Vol. 12, No. 1, pp. 105–114, July, 1972.