Dollhopf, John, Ian Goulden, and Curtis Greene. “Words Avoiding a Reflexive Acyclic Relation”. The Electronic Journal of Combinatorics 11, no. 2 (February 8, 2006): #R28. Accessed May 13, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v11i2r28.