Team, S. “Regularity of the Semigroups of Transformations With a Fixed Point Set: Ronnason Chinram, Winita Yonthanthum”. Thai Journal of Mathematics, vol. 18, no. 3, Sept. 2020, pp. 1261–1268, https://thaijmath2.in.cmu.ac.th/index.php/thaijmath/article/view/1070.