Representation theory of monoids consisting of order-preserving functions and order-reversing functions on an n-set

Abstract

Let $\operatorname{OD}_{n}$ be the monoid of all order-preserving functions and order-reversing functions on the set $\{1,\ldots,n\}$. We describe a quiver presentation for the monoid algebra $\Bbbk\operatorname{OD}_{n}$ where $\Bbbk$ is a field whose characteristic is not 2. We show that the quiver consists of two straightline paths, one with $n-1$ vertices and one with $n$ vertices, and that all compositions of consecutive arrows are equal to $0$. As part of the proof we obtain a complete description of all homomorphisms between induced left Sch\"utzenberger modules of $\Bbbk\operatorname{OD}_{n}$. We also define $\operatorname{COD}_{n}$ to be a covering of $\operatorname{OD}_{n}$ with an artificial distinction between order-preserving and order-reversing constant functions. We show that $\operatorname{COD}_{n}\simeq\operatorname{O}_{n}\rtimes\mathbb{Z}_{2}$ where $\operatorname{O}_{n}$ is the monoid of all order-preserving functions on the set $\{1,\ldots,n\}$. Moreover, if $\Bbbk$ is a field whose characteristic is not $2$ we prove that $\Bbbk\operatorname{COD}_{n}\simeq\Bbbk\operatorname{O}_{n}\times\Bbbk\operatorname{O}_{n}$. As a corollary, we deduce that the quiver of $\Bbbk\operatorname{COD}_{n}$ consists of two straightline paths with n vertices, and that all compositions of consecutive arrows are equal to $0$.