A Coxeter type classification of Dynkin type $\mathbb{A}_n$ non-negative posets

Abstract

We continue the Coxeter spectral analysis of finite connected posets $I$ that are non-negative in the sense that their symmetric Gram matrix $G_I:=\frac{1}{2}(C_I + C_I^{tr})\in\mathbb{M}_{m}(\mathbb{Q})$ is positive semi-definite of rank $n\geq 0$, where $C_I\in\mathbb{M}_m(\mathbb{Z})$ is the incidence matrix of $I$ encoding the relation $\preceq_I$. We extend the results of [Fundam. Inform., 139.4(2015), 347--367] and give a complete Coxeter spectral classification of finite connected posets $I$ of Dynkin type $\mathbb{A}_n$. We show that such posets $I$, with $|I|>1$, yield exactly $\lfloor\frac{m}{2}\rfloor$ Coxeter types, one of which describes the positive (i.e., with $n=m$) ones. We give an exact description and calculate the number of posets of every type. Moreover, we prove that, given a pair of such posets $I$ and $J$, the incidence matrices $C_I$ and $C_J$ are $\mathbb{Z}$-congruent if and only if $\mathbf{specc}_I = \mathbf{specc}_J$, and present deterministic algorithms that calculate a $\mathbb{Z}$-invertible matrix defining such a $\mathbb{Z}$-congruence in a polynomial time.