Finite-Monoid Compression in Syntactic Concept Lattices: Arity Hierarchies and a Pseudovariety Trichotomy

Clark’s syntactic concept lattice (SCL) records two-sided distributional structure, and Wurm extended it to tuples of arbitrary finite arity. We study ${\mathrm{cmp}}_f(L)$, the minimum image size of a finite-monoid observation that preserves guarded tuple substitution through arity $f$ on the principal layer. For regular languages, we characterize ${\mathrm{cmp}}_f(L)$ exactly as the least cardinality of the codomain of an $f$-separating relational morphism from the pointed syntactic monoid. Let $\mathrm{ch}(\mathbf{V})$ denote the least arity at which these compression numbers stabilize uniformly over a pseudovariety $\mathbf{V}$. Our main result is the following trichotomy of possible uniform heights: $\mathrm{ch}(\mathbf{V})\in \{1,2,\mathrm{\infty }\}$, with $\mathrm{ch}(\mathbf{V})=\mathrm{\infty }$ if and only if $\mathrm{Synt}(\{ab\})\in \mathbf{V}$. Thus no finite uniform compression height $3,4,\dots$ occurs. The infinite case is sharp: inside $⟨\mathrm{Synt}(\{ab\})⟩$, every boundary $d\to d+1$ admits unbounded compression gaps, and arbitrary finite strict prefixes of the arity hierarchy are realizable. On the finite side, commutative monoids and bands stabilize at arity one, while every completely regular syntactic monoid stabilizes by arity two; finite group kernels show that the binary bound is sharp. At unary arity, every nonempty finite simple graph is realized by an explicit length-three language, yielding an exact chromatic-number formula and NP-completeness of deciding ${\mathrm{cmp}}_1(L)\le 3$ for explicitly listed length-three languages. The structural boundary between compression heights one and two remains open.