The Automorphism Group of the Subsequence Poset B m,n

In this paper we are interested in the automorphism group of the poset B _{m, n} . B _{m, n} constitutes the words obtained from the cyclic word of length n on an alphabet of m letters in by deleting on all possible ways and their natural order. We prove: Résumé: Le but de ce papier est la détermination du groupe d"automorphismes des ordres B _{m, n} . Il s"agit des mots obtenus à partir du mot cyclique de longeur n sur un alphabet de m lettres par suppression successive de lettres et ordonnés naturellement. On prouve: AutB _{m, n} = {S _n for 1 n m, S ₂ S _2m-n for m + 1n 2m - 1, S ₂for 2m n.     

Cartesian products of trees and paths

We characterize the (weak) Cartesian products of trees among median graphs by a forbidden 5-vertex convex subgraph. The number of tree factors (if finite) is half the length of a largest isometric cycle. Then a characterization of Cartesian products of n trees obtains in terms of isometric cycles and intervals. Finally we investigate to what extent the proper intervals determine the product structure. © 1996 John Wiley & Sons, Inc.     

The poset of closures as a model of changing databases

The combinatorial properties of the poset of closures are studied, especially the degrees in the Hasse diagram.The research of these authors was supported by the Hungarian National Foundation for Scientific Research, grant numbers 1812 and 700021.