BOKAYEV, N.A.; SYZDYKOVA, A.T. 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, [S. l.], v. 78, n. 2, p. 31–36, 2015. Disponível em: http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/19. Acesso em: 7 sep. 2025.