Team, S. “Characterization of Digraphs of Right (Left) Zero Unions of Groups: Sr. Arworn, U. Knauer, N. Na Chiangmai”. Thai Journal of Mathematics, vol. 1, no. 1, June 2003, pp. 131-40, https://thaijmath2.in.cmu.ac.th/index.php/thaijmath/article/view/15.