Gonska, Heiner, and Ralitza K. Kovacheva. “The Second Order Modulus Revisited: Remarks, Applications, Problems”. J. Numer. Anal. Approx. Theory 53, no. 1 (July 11, 2024): 78–102. Accessed April 12, 2025. https://ictp.acad.ro/jnaat/journal/article/view/1410.