Derech, V. D. “About Permutable Inverse Semigroups of a Finite Rank”. Visnyk of Vinnytsia Politechnical Institute, no. 4 (November 12, 2010): 116–118. Accessed July 16, 2024. https://visnyk.vntu.edu.ua/index.php/visnyk/article/view/213.