Berţi, Şt. N. “On the Generalized Sum of Intervals”. Rev. Anal. Numér. Théorie Approximation 3, no. 1 (February 1, 1974): 11–21. Accessed May 19, 2024. https://ictp.acad.ro/jnaat/journal/article/view/1974-vol3-no1-art2.