D. . Leviatan, O. V. Motorna, and I. A. Shevchuk, “No Jackson-type estimates for piecewise $q$-monotone, $q\ge3$, trigonometric approximation”, Ukr. Mat. Zhurn., vol. 74, no. 5, pp. 662–675, Jun. 2022, doi: 10.37863/umzh.v74i5.7081.