Jürgensen, Helmut, and Ian McQuillan. “Homomorphisms Preserving Types of Density”. Acta Cybernetica 19, no. 2 (January 1, 2009): 499–516. Accessed May 9, 2025. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3779.