Grigorian, S. A., and T. A. Khorkova. “On $w$-Extensions of the Abelian Semigroups. Armenian Journal of Mathematics”. Armenian Journal of Mathematics 2, no. 2 (June 4, 2009): 73–76. Accessed April 3, 2025. https://armjmath.sci.am/index.php/ajm/article/view/54.