G. Endimioni, “Hopficity and Co-Hopficity in Soluble Groups”, Ukr. Mat. Zhurn., vol. 56, no. 10, pp. 1335–1341, Oct. 2004, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3846