We introduce the package Posets for Macaulay2. This package provides a data structure and the necessary methods for working with partially ordered sets, also called posets. In particular, the package implements methods to enumerate many commonly studied classes of posets, perform operations on posets, and calculate various invariants associated to posets.
INTRODUCTION.A partial order is a binary relation over a set P that is antisymmetric, reflexive, and transitive. A set P together with a partial order is called a poset, or partially ordered set. We refer the reader to the seminal text [Stanley 2012] for definitions omitted herein.Posets are combinatorial structures that are used in modern mathematical research, particularly in algebra. We introduce the package Posets for Macaulay2 [Grayson and Stillman] via three distinct posets or related ideals which arise naturally in combinatorial algebra.We first describe two posets that are generated from algebraic objects. The intersection semilattice associated to a hyperplane arrangement can be used to compute the number of unbounded and bounded real regions cut out by a hyperplane arrangement, as well as the dimensions of the homologies of the complex complement of a hyperplane arrangement.Given a monomial ideal, the lcm-lattice of its minimal generators gives information on the structure of the free resolution of the original ideal. Specifically, two monomial ideals with isomorphic lcm-lattices have the "same" (up to relabeling) minimal free resolution, and the lcm-lattice can be used to compute, among other things, the multigraded Betti numbers