Gonska, Heinz H., and Gancho T. Tachev. “The Second Ditzian-Totik Modulus Revisited: Refined Estimates for Positive Linear Operators”. Rev. Anal. Numér. Théor. Approx., vol. 32, no. 1, Feb. 2003, pp. 39-61, https://doi.org/10.33993/jnaat321-733.