Ochem, Pascal, and Matthieu Rosenfeld. “Avoidability of Formulas With Two Variables”. The Electronic Journal of Combinatorics 24, no. 4 (November 3, 2017): P4.30. Accessed March 16, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v24i4p30.