3,169 research outputs found
Counting Permutations Modulo Pattern-Replacement Equivalences for Three-Letter Patterns
We study a family of equivalence relations on , the group of
permutations on letters, created in a manner similar to that of the Knuth
relation and the forgotten relation. For our purposes, two permutations are in
the same equivalence class if one can be reached from the other through a
series of pattern-replacements using patterns whose order permutations are in
the same part of a predetermined partition of .
When the partition is of and has one nontrivial part and that part is
of size greater than two, we provide formulas for the number of classes created
in each previously unsolved case. When the partition is of and has two
nontrivial parts, each of size two (as do the Knuth and forgotten relations),
we enumerate the classes for of the unresolved cases. In two of these
cases, enumerations arise which are the same as those yielded by the Knuth and
forgotten relations. The reasons for this phenomenon are still largely a
mystery
Generating functions for generating trees
Certain families of combinatorial objects admit recursive descriptions in
terms of generating trees: each node of the tree corresponds to an object, and
the branch leading to the node encodes the choices made in the construction of
the object. Generating trees lead to a fast computation of enumeration
sequences (sometimes, to explicit formulae as well) and provide efficient
random generation algorithms. We investigate the links between the structural
properties of the rewriting rules defining such trees and the rationality,
algebraicity, or transcendence of the corresponding generating function.Comment: This article corresponds, up to minor typo corrections, to the
article submitted to Discrete Mathematics (Elsevier) in Nov. 1999, and
published in its vol. 246(1-3), March 2002, pp. 29-5
GL(p) x GL(q)-orbit closures on the flag variety and Schubert structure constants for (p,q)-pairs
We give positive combinatorial descriptions of Schubert structure constants
for the full flag variety in type when and form
what we refer to as a "-pair" (). The key observation is that a
certain subset of the -orbit closures
on the flag variety (those satisfying an easily stated pattern avoidance
condition) are Richardson varieties. The result on structure constants follows
when one combines this observation with a theorem of Brion concerning
intersection numbers of spherical subgroup orbit closures and Schubert
varieties.Comment: This paper is now replaced by arXiv:1209.0739. I am leaving this here
only because the published version of arXiv:1109.2574 refers to this version,
and specific results therei
Combinatorial generation via permutation languages
In this work we present a general and versatile algorithmic framework for exhaustively generating a large variety of different combinatorial objects, based on encoding them as permutations.
This approach provides a unified view on many known results and allows us to prove many new ones.
In particular, we obtain the following four classical Gray codes as special cases: the Steinhaus-Johnson-Trotter algorithm to generate all permutations of an -element set by adjacent transpositions; the binary reflected Gray code to generate all -bit strings by flipping a single bit in each step; the Gray code for generating all -vertex binary trees by rotations due to Lucas, van Baronaigien, and Ruskey; the Gray code for generating all partitions of an -element ground set by element exchanges due to Kaye.
We present two distinct applications for our new framework:
The first main application is the generation of pattern-avoiding permutations, yielding new Gray codes for different families of permutations that are characterized by the avoidance of certain classical patterns, (bi)vincular patterns, barred patterns, Bruhat-restricted patterns, mesh patterns, monotone and geometric grid classes, and many others.
We thus also obtain new Gray code algorithms for the combinatorial objects that are in bijection to these permutations, in particular for five different types of geometric rectangulations, also known as floorplans, which are divisions of a square into rectangles subject to certain restrictions.
The second main application of our framework are lattice congruences of the weak order on the symmetric group~.
Recently, Pilaud and Santos realized all those lattice congruences as -dimensional polytopes, called quotientopes, which generalize hypercubes, associahedra, permutahedra etc.
Our algorithm generates the equivalence classes of each of those lattice congruences, by producing a Hamilton path on the skeleton of the corresponding quotientope, yielding a constructive proof that each of these highly symmetric graphs is Hamiltonian.
We thus also obtain a provable notion of optimality for the Gray codes obtained from our framework: They translate into walks along the edges of a polytope
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