11,863 research outputs found
Overview of the Heisenberg--Weyl Algebra and Subsets of Riordan Subgroups
In a first part, we are concerned with the relationships between polynomials
in the two generators of the algebra of Heisenberg--Weyl, its Bargmann--Fock
representation with differential operators and the associated one-parameter
group.Upon this basis, the paper is then devoted to the groups of Riordan
matrices associated to the related transformations of matrices (i.e.
substitutions with prefunctions). Thereby, various properties are studied
arising in Riordan arrays, in the Riordan group and, more specifically, in the
`striped' Riordan subgroups; further, a striped quasigroup and a semigroup are
also examined. A few applications to combinatorial structures are also briefly
addressed in the Appendix.Comment: Version 3 of the paper entitled `On subsets of Riordan subgroups and
Heisenberg--Weyl algebra' in [hal-00974929v2]The present article is published
in The Electronic Journal of Combinatorics, Volume 22, Issue 4, 40 pages
(Oct. 2015), pp.Id: 1
Partitioning random graphs into monochromatic components
Erd\H{o}s, Gy\'arf\'as, and Pyber (1991) conjectured that every -colored
complete graph can be partitioned into at most monochromatic components;
this is a strengthening of a conjecture of Lov\'asz (1975) in which the
components are only required to form a cover. An important partial result of
Haxell and Kohayakawa (1995) shows that a partition into monochromatic
components is possible for sufficiently large -colored complete graphs.
We start by extending Haxell and Kohayakawa's result to graphs with large
minimum degree, then we provide some partial analogs of their result for random
graphs. In particular, we show that if , then a.a.s. in every -coloring of there exists
a partition into two monochromatic components, and for if , then a.a.s. there exists an -coloring
of such that there does not exist a cover with a bounded number of
components. Finally, we consider a random graph version of a classic result of
Gy\'arf\'as (1977) about large monochromatic components in -colored complete
graphs. We show that if , then a.a.s. in every
-coloring of there exists a monochromatic component of order at
least .Comment: 27 pages, 2 figures. Appears in Electronic Journal of Combinatorics
Volume 24, Issue 1 (2017) Paper #P1.1
A Discontinuity in the Distribution of Fixed Point Sums
The quantity , defined as the number of permutations of the set
whose fixed points sum to , shows a sharp discontinuity
in the neighborhood of . We explain this discontinuity and study the
possible existence of other discontinuities in for permutations. We
generalize our results to other families of structures that exhibit the same
kind of discontinuities, by studying when ``fixed points'' is replaced
by ``components of size 1'' in a suitable graph of the structure. Among the
objects considered are permutations, all functions and set partitions.Comment: 1 figur
On the Spectrum of the Derangement Graph
We derive several interesting formulae for the eigenvalues of the derangement graph and use them to settle affirmatively a conjecture of Ku regarding the least eigenvalue
Motzkin paths, Motzkin polynomials and recurrence relations
We consider the Motzkin paths which are simple combinatorial objects appearing in many contexts. They are counted by the Motzkin numbers, related to the well known Catalan numbers. Associated with the Motzkin paths, we introduce the Motzkin polynomial, which is a multi-variable polynomial "counting" all Motzkin paths of a certain type. Motzkin polynomials (also called Jacobi-Rogers polynomials) have been studied before, but here we deduce sonic properties based on recurrence relations. The recurrence relations proved here also allow an efficient computation of the Motzkin polynomials. Finally, we show that the matrix entries of powers of an arbitrary tridiagonal matrix are essentially given by Motzkin polynomials, a property commonly known but usually stated without proof
Why Delannoy numbers?
This article is not a research paper, but a little note on the history of
combinatorics: We present here a tentative short biography of Henri Delannoy,
and a survey of his most notable works. This answers to the question raised in
the title, as these works are related to lattice paths enumeration, to the
so-called Delannoy numbers, and were the first general way to solve Ballot-like
problems. These numbers appear in probabilistic game theory, alignments of DNA
sequences, tiling problems, temporal representation models, analysis of
algorithms and combinatorial structures.Comment: Presented to the conference "Lattice Paths Combinatorics and Discrete
Distributions" (Athens, June 5-7, 2002) and to appear in the Journal of
Statistical Planning and Inference
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