Pattern Avoidance for Fibonacci Sequences using $k$-Regular Words
Emily Downing, Elizabeth Hartung, Cody Lucido, Aaron Williams
TL;DR
This work studies pattern avoidance in $k$-regular words, connecting combinatorial word structures to generalized Fibonacci recurrences. It provides a simple bijective proof that Fibonacci-$k$ words are counted by $a_k(n)$ when avoiding $\{121,123,132,213\}$, and similarly shows that $k$-Fibonacci words are counted by $b_k(n)$ when avoiding $\{122,213\}$ for $k\ge2$, with precise base cases. It further proves a vincular-pattern extension where the Fibonacci-squared words are enumerated by $a_1(n)^2$, via a detailed annex/base decomposition and a prefix-tree analysis. Together, these results extend connections among Fibonacci-type sequences, Stirling/regular word pattern avoidance, and classical combinatorial families, while opening questions about broader multi-pattern avoidance in $k$-regular words.
Abstract
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.