Almeida, Jorge, Zoltán Ésik, and Jean-Éric Pin. “Commutative Positive Varieties of Languages”. Acta Cybernetica 23, no. 1 (January 1, 2017): 91-111. Accessed April 20, 2024. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3917.