Gupta, S., and W. Holubowski. “On 2-Symmetric Words in Nilpotent Groups”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 7 (July 25, 2002): 1020–1024. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4142.