Abelian maximal pattern complexity and extremal words
We study the Abelian maximal pattern complexity $p_{α}^{\ast \mathrm{ab}}(k)$ of infinite words over finite alphabets. For recurrent aperiodic words using exactly $\ell\geq 2$ letters, we prove the sharp lower bound $p_{α}^{\ast \mathrm{ab}}(k)\geq k+\ell-1$ and give a structural characterization of those words attaining equality for every $k\geq 1$. Without assuming recurrence, we prove that an infinite word $α$ is aperiodic if and only if $\binom{p_{α}^{\ast \mathrm{ab}}(k)}{2}\geq k$ for every positive integer $k$. For aperiodic words over $\ell\geq 2$ letters, each occurring infinitely often, we further prove that $p_α^{\ast \mathrm{ab}}(k)\geq m$ whenever $\binom{m}{2}\leq(\ell-1)(k-\ell+2)$, for all positive integers $m,k$. Together with a matching construction, this shows that the minimum Abelian maximal pattern complexity in this class is $\sqrt{2(\ell-1)k}+O_{\ell}(1)$. We call a word an Abelian pattern Sturmian word if, for every positive integer $k$, its Abelian maximal pattern complexity is the least positive integer $m$ satisfying $\binom{m}{2}\geq k$. We show that a word is Abelian pattern Sturmian if and only if, after relabeling its alphabet, it is the characteristic word of an infinite set $E\subset\mathbb{N}_0$ whose bipartite incidence graph is a forest. This graph has two disjoint copies of $\mathbb{N}_0$ as its vertex classes, with a left vertex $r$ adjacent to a right vertex $s$ exactly when $r+s\in E$.