Leviatan , D., et al. “No Jackson-Type Estimates for Piecewise $q$-Monotone, $q\ge3$, Trigonometric Approximation”. Ukrains’kyi Matematychnyi Zhurnal, vol. 74, no. 5, June 2022, pp. 662-75, https://doi.org/10.37863/umzh.v74i5.7081.