Demetrovics, János, and Vu Duc Thi. 1993. “Some Problems Concerning Armstrong Relations of Dual Schemes and Relation Schemes in the Relational Datamodel”. Acta Cybernetica 11 (1-2), 35-47. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3419.