\((k,\lambda)\)-anti-powers and other patterns in words

Amanda Burcroff¹

¹University of Michigan

The electronic journal of combinatorics, Tome 25 (2018) no. 4

Résumé

Given a word, we are interested in the structure of its contiguous subwords split into $k$ blocks of equal length, especially in the homogeneous and anti-homogeneous cases. We introduce the notion of $(\mu_1,\dots,\mu_k)$-block-patterns, words of the form $w = w_1\cdots w_k$ where, when $\{w_1,\dots,w_k\}$ is partitioned via equality, there are $\mu_s$ sets of size $s$ for each $s \in \{1,\dots,k\}$. This is a generalization of the well-studied $k$-powers and the $k$-anti-powers recently introduced by Fici, Restivo, Silva, and Zamboni, as well as a refinement of the $(k,\lambda)$-anti-powers introduced by Defant. We generalize the anti-Ramsey-type results of Fici et al. to $(\mu_1,\dots,\mu_k)$-block-patterns and improve their bounds on $N_\alpha(k,k)$, the minimum length such that every word of length $N_\alpha(k,k)$ on an alphabet of size $\alpha$ contains a $k$-power or $k$-anti-power. We also generalize their results on infinite words avoiding $k$-anti-powers to the case of $(k,\lambda)$-anti-powers. We provide a few results on the relation between $\alpha$ and $N_\alpha(k,k)$ and find the expected number of $(\mu_1,\dots,\mu_k)$-block-patterns in a word of length $n$.

Détail

DOI : 10.37236/8032

Classification : 68R15
Mots-clés : anti-power, block-pattern, anti-Ramsey