$(k,\lambda)$-Anti-Powers and Other Patterns in Words

  • Amanda Burcroff
Keywords: Anti-power, Block-pattern, Anti-Ramsey


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 $n$.
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