Burstein, Alexander, and Toufik Mansour. “Words Restricted by Patterns With at Most 2 Distinct Letters”. The Electronic Journal of Combinatorics 9, no. 2 (October 31, 2002): #R3. Accessed May 29, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v9i2r3.