1. P. P. Korovkin, ?Asymptotic properties of positive methods for the summation of Fourier series,? Usp. Matem. Nauk,15, No. 1 (91) (1960).
2. É. N. Morozov, ?Convergence of linear positive operators in the space of continuous 2?-periodic functions of two variables,? Uch. Zap. Kalinin. Gos. Ped. Inst.,26 (1958).
3. V. P. Bugaets, ?Approximation of functions of two variables by linear positive polynomial operators,? Studies in Contemporary Problems in the Summation and Approximation of Functions and Their Applications [in Russian], Dnepropetrovsk (1967).