Bokayev N, Syzdykova A. An estimate of best approximation by angle of sum of double series with respect to multiplicative system with quasiconvex coefficients . M [Internet]. 2015 June 29 [cited 2026 Oct. 27];2(78):31-6. Available from: https://mts.buketov.edu.kz/mathematics-vestnik/article/view/19