Abubakar, Rafiat Bada. “A Note on Free Semigroups”. Faculty of Natural and Applied Sciences Journal of Mathematical Modeling and Numerical Simulation 2, no. 2 (March 31, 2025): 1–9. Accessed December 8, 2025. https://www.fnasjournals.com/index.php/FNAS-JMNS/article/view/761.