Zhuchok, A. V., and J. Koppitz. “Free Products of $n$-Tuple Semigroups”. Ukrains’kyi Matematychnyi Zhurnal 70, no. 11 (November 25, 2018): 1484-1498. Accessed April 20, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1652.