Emily Downing ; Elizabeth Hartung ; Cody Lucido ; Aaron Williams - Pattern Avoidance for Fibonacci Sequences using $k$-Regular Words

dmtcs:12752 - Discrete Mathematics & Theoretical Computer Science, January 20, 2026, vol. 26:1, Permutation Patterns 2023 - https://doi.org/10.46298/dmtcs.12752
Pattern Avoidance for Fibonacci Sequences using $k$-Regular WordsArticle

Authors: Emily Downing ; Elizabeth Hartung ; Aaron Williams

Two $k$-ary Fibonacci recurrences are $a_k(n) = a_k(n-1) + k \cdot a_k(n-2)$ and $b_k(n) = k \cdot b_k(n-1) + b_k(n-2)$. We provide a simple proof that $a_k(n)$ is the number of $k$-regular words over $[n] = \{1,2,\ldots,n\}$ that avoid patterns $\{121, 123, 132, 213\}$ when using base cases $a_k(0) = a_k(1) = 1$ for any $k \geq 1$. This was previously proven by Kuba and Panholzer in the context of Wilf-equivalence for restricted Stirling permutations, and it creates Simion and Schmidt's classic result on the Fibonacci sequence when $k=1$, and the Jacobsthal sequence when $k=2$. We complement this theorem by proving that $b_k(n)$ is the number of $k$-regular words over $[n]$ that avoid $\{122, 213\}$ with $b_k(0) = b_k(1) = 1$ for any~$k \geq 2$. Finally, we conjecture that $|Av^{2}_{n}(\underline{121}, 123, 132, 213)| = a_1(n)^2$ for $n \geq 0$. That is, vincularizing the Stirling pattern in Kuba and Panholzer's Jacobsthal result gives the Fibonacci-squared numbers.

20 pages, submitted to special journal issue for Permutation Patterns 2023 (PP23) in DMTCS


Volume: vol. 26:1, Permutation Patterns 2023
Section: Special issues
Published on: January 20, 2026
Accepted on: May 15, 2025
Submitted on: December 28, 2023
Keywords: Combinatorics, Discrete Mathematics, 05 (Primary) 68 (Secondary), G.2.1; G.4

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