A Modular Structure Theorem for Minimal Periodic Decompositions and Periodicity of Configurations with $P_{\eta }(4,n)\le 4n$

Abstract

Nivat's conjecture asserts that every two-dimensional configuration $\eta :{\mathbb{Z}}^2\to \mathcal{A}$ whose rectangular pattern complexity satisfies $P_{\eta }(k,n)\le kn$ for some $k,n\in \mathbb{N}$ is periodic. A theorem of Cyr and Kra \cite{CyrKra16} establishes the conjecture in the short-rectangle case $P_{\eta }(k,n)\le kn$, with $k\le 3$. Using the algebraic framework of Kari-Szabados \cite{KariSzabados20} and recent advances on periodic decompositions and one-sided nonexpansive directions \cite{Colle23,Colle22}, we extend the Cyr-Kra result to the case $P_{\eta }(4,n)\le 4n$: every configuration satisfying this complexity bound is periodic. The key new ingredient is an intermediate structural theorem of independent interest: for any non-periodic configuration with low convex pattern complexity and integer-valued alphabet $\mathcal{A}$ contained in ${\mathbb{Z}}_{+}$, there exist a configuration $\vartheta$ in the orbit closure of $\eta$, a $\mathbb{Z}$-minimal periodic decomposition $\vartheta ={\vartheta }_1+\cdots +{\vartheta }_m$, a prime $p\in \mathbb{N}$ with $\mathcal{A}\subset [[p]]$, and pairs of disjoint half-planes $U_i,V_i\subset {\mathbb{Z}}^2$ such that the reductions modulo $p$ of the components ${\vartheta }_i$ are fully periodic on $U_i$ and on $V_i$ simultaneously, for each $1\le i\le m$.

arXiv comment26 pages