Properties of congruences of twisted partition monoids and their lattices

Abstract

We build on the recent characterisation of congruences on the infinite twisted partition monoids $\mathcal{P}_{n}^\Phi$ and their finite $d$-twisted homomorphic images $\mathcal{P}_{n,d}^\Phi$, and investigate their algebraic and order-theoretic properties. We prove that each congruence of $\mathcal{P}_{n}^\Phi$ is (finitely) generated by at most $\lceil\frac{5n}2\rceil$ pairs, and we characterise the principal ones. We also prove that the congruence lattice $\textsf{Cong}(\mathcal{P}_{n}^\Phi)$ is not modular (or distributive); it has no infinite ascending chains, but it does have infinite descending chains and infinite antichains. By way of contrast, the lattice $\textsf{Cong}(\mathcal{P}_{n,d}^\Phi)$ is modular but still not distributive for $d>0$, while $\textsf{Cong}(\mathcal{P}_{n,0}^\Phi)$ is distributive. We also calculate the number of congruences of $\mathcal{P}_{n,d}^\Phi$, showing that the array $\big(|\textsf{Cong}(\mathcal{P}_{n,d}^\Phi)|\big)_{n,d\geq 0}$ has a rational generating function, and that for a fixed $n$ or $d$, $|\textsf{Cong}(\mathcal{P}_{n,d}^\Phi)|$ is a polynomial in $d$ or $n\geq 4$, respectively.