Going down in (semi)lattices of finite Moore families and convex geometries

International audienceIn this paper we first study the changes occuring in the posets of irreducible elements when one goes from an arbitrary Moore family (respectively, a convex geometry) to one of its lower covers in the lattice of all Moore families (respectively, in the semilattice of all convex geometries) defined on a finite set. Then, we show that the poset of all convex geometries that have the same poset of join-irreducible elements is a ranked join-semilattice, and we give an algorithm for computing it. Finally, we prove that the lattice of all ideals of a given poset P is the only convex geometry having a poset of join-irreducible elements isomorphic to P if and only if the width of P is less than 3.Dans ce texte, nous étudions d'abord les changements dans les ensembles ordonnés d'éléments irréductibles lorsqu'on passe d'une famille de Moore arbitraire (respectivement, d'une géométrie convexe) à l'une de ses couvertures inférieures dans le treillis de toutes les familles de Moore (respectivement, dans le demi-treillis des géométries convexes). Nous montrons ensuite que l'ensemble ordonné de toutes les géométries convexes ayant le même ensemble ordonné d'éléments sup-irréductibles est un demi-treillis rangé et nous donnons un algorithme pour le calculer. Enfin nous caractérisons les ensembles ordonnés P pour lesquels le treillis de leurs idéaux est l'unique géométrie convexe ayant son ensemble ordonné d'éléments sup-irréductibles isomorphe à P

A characterization for all interval doubling schemes of the lattice of permutations

The lattice \textbfS_n of all permutations on a n-element set has been shown to be \emphbounded [CAS], which is a strong constructive property characterized by the fact that \textbfS_n admits what we call an \emph interval doubling scheme. In this paper we characterize all interval doubling schemes of the lattice \textbfS_n, a result that gives a nice precision on the bounded nature of the lattice of permutations. This theorem is a direct corollary of two strong properties that are also given with their proofs

Des chaînes et des antichaînes dans les ensembles ordonnés finis

L’un des nombreux domaines dans lesquels Bruno Leclerc a travaillé et publié est celui des ensembles ordonnés. Plus précisément, il s’est fortement intéressé à des propriétés d’ensembles ordonnés relatives à des sous-structures bien particulières, les chaînes et les antichaînes. Beaucoup de problèmes de tri, de recherche et d’ordonnancement que l’on rencontre par exemple en informatique et en recherche opérationnelle, sont liés à la détermi-nation du cardinal maximum d’une antichaîne d’un ensemble ordonné, c’est-à-dire de sa largeur.Cet article se repenche sur ces centres d’intérêts de l’oeuvre de Bruno, en rappelant d’une part certains grands théorèmes classiques relatifs à ces notions et, d’autre part, des résultats de Bruno sur ces sujets. Nos développements se restreignent au cas fini.One of the numerous fields investigated by Bruno Leclerc is the theory of finite posets. More precisely, he was very interested in properties of posets relative to some particular substructures, the so-called chains and antichains. Many problems in sorting, search and scheduling, that one can find for instance in computer science and operational research, are connected with the computing of the maximum cardinality of an antichain of a poset, that is, of its width.This paper looks into those centers of interest of Bruno’s work, recalling on one hand some strong and classical theorems relating to these notions and, on the other hand, some results by Bruno on these subjects. Our developments only concern the finite case

Going down in (semi)lattices of finite Moore families and convex geometries

In this paper we first study the changes occuring in the posets of irreducible elements when one goes from an arbitrary Moore family (respectively, a convex geometry) to one of its lower covers in the lattice of all Moore families (respectively, in the semilattice of all convex geometries) defined on a finite set. Then, we show that the poset of all convex geometries that have the same poset of join-irreducible elements is a ranked join-semilattice, and we give an algorithm for computing it. Finally, we prove that the lattice of all ideals of a given poset P is the only convex geometry having a poset of join-irreducible elements isomorphic to P if and only if the width of P is less than 3.closure system;convex geometry;cover relation;join-irreducible;Moore family;poset of irreducible;semilattice

Some lattices of closure systems on a finite set

In this paper we study two lattices of significant particular closure systems on a finite set, namely the union stable closure systems and the convex geometries. Using the notion of (admissible) quasi-closed set and of (deletable) closed set, we determine the covering relation \prec of these lattices and the changes induced, for instance, on the irreducible elements when one goes from C to C' where C and C' are two such closure systems satisfying C \prec C'. We also do a systematic study of these lattices of closure systems, characterizing for instance their join-irreducible and their meet-irreducible elements

Cayley lattices of finite Coxeter groups are bounded

AbstractAn interval doubling is a constructive operation which applies on a poset P and an interval I of P and constructs a new “bigger” poset P′=P[I] by replacing in P the interval I with its direct product with the two-element lattice. The main contribution of this paper is to prove that finite Coxeter lattices are bounded, i.e., that they can be constructed starting with the two-element lattice by a finite series of interval doublings.The boundedness of finite Coxeter lattices strengthens their algebraic property of semidistributivity. It also brings to light a relation between the interval doubling construction and the reflections of Coxeter groups.Our approach to the question is somewhat indirect. We first define a new class HH of lattices and prove that every lattice of HH is bounded. We then show that Coxeter lattices are in HH and the theorem follows. Another result says that, given a Coxeter lattice LW and a parabolic subgroup WH of the finite Coxeter group W, we can construct LW starting from WH by a series of interval doublings. For instance the lattice, associated with An, of all the permutations on n+1 elements is obtained from An−1 by a series of interval doublings. The same holds for the lattices associated with the other infinite families of Coxeter groups Bn, Dn and I2(n)

A characterization for all interval doubling schemes of the lattice of permutations

The lattice \textbfS_n of all permutations on a n-element set has been shown to be \emphbounded [CAS], which is a strong constructive property characterized by the fact that \textbfS_n admits what we call an \emph interval doubling scheme. In this paper we characterize all interval doubling schemes of the lattice \textbfS_n, a result that gives a nice precision on the bounded nature of the lattice of permutations. This theorem is a direct corollary of two strong properties that are also given with their proofs

A Characterization for All Interval Doubling Schemes of the Lattice of Permutations

this paper we characterize all interval doubling schemes of the lattic