Bokayev, N.A., and A.T. Syzdykova. “An Estimate of Best Approximation by Angle of Sum of Double Series With Respect to Multiplicative System With Quasiconvex Coefficients ”. Bulletin of the Karaganda University. Mathematics Series 78, no. 2 (June 29, 2015): 31–36. Accessed September 7, 2025. http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/19.