Leviatan , D., Motorna, O. V., & Shevchuk, I. A. (2022). No Jackson-type estimates for piecewise $q$-monotone, $q\ge3$, trigonometric approximation. Ukrains’kyi Matematychnyi Zhurnal, 74(5), 662 - 675. https://doi.org/10.37863/umzh.v74i5.7081