E. G. Usenko, “Coincidence criteria for the kernel of a function and the kernel of its integral almost positive means”, Ukr. Mat. Zhurn., vol. 51, no. 9, pp. 1267–1275, Sep. 1999, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4722