Aigner, Mats, Vitalij A. Chatyrko, and Venuste Nyagahakwa. “The Algebra of Semigroups of Sets”. MATHEMATICA SCANDINAVICA 116, no. 2 (June 26, 2015): 161–170. Accessed April 18, 2024. https://www.mscand.dk/article/view/21158.