Kubica, Bartłomiej Jacek. “The ‘Second Derivative’ of a Non-Differentiable Function and Its Use in Interval Optimization Methods”. Journal of Telecommunications and Information Technology 46, no. 4 (December 30, 2011): 81–85. Accessed October 9, 2025. https://www.jtit.pl/jtit/article/view/1182.