2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/119248The main object of the present paper is to give a complete result regarding the best approximation rate of certain trigonometric series in general complex valued continuous function space under a new condition which gives an essential generalization to $O$-regularly varying quasimonotonicity. An application is present in Section 3.14 pagesClassical Analysis and ODEs42A20; 42A32text