Showing papers in "Electronic Journal of Combinatorics in 2002"

The Number of Labeled 2-Connected Planar Graphs

Abstract: We derive the asymptotic expression for the number of labeled 2-connected planar graphs with respect to vertices and edges. We also show that almost all such graphs with $n$ vertices contain many copies of any fixed planar graph, and this implies that almost all such graphs have large automorphism groups.

There are ternary circular square-free words of length n for n 18.

Abstract: There are circular square-free words of length $n$ on three symbols for $n\ge 18$. This proves a conjecture of R. J. Simpson.

Identifying Codes with Small Radius in Some Infinite Regular Graphs

Abstract: Let G =( V;E) be a connected undirected graph and S a subset of vertices. If for all vertices v 2 V , the sets Br(v) \ S are all nonempty and dierent, where Br(v) denotes the set of all points within distance r from v ,t hen we callS an r-identifying code. We give constructive upper bounds on the best possible density of r-identifying codes in four innite regular graphs, for small values of r.

A Generalisation of Transversals for Latin Squares

Abstract: We define a $k$-plex to be a partial latin square of order $n$ containing $kn$ entries such that exactly $k$ entries lie in each row and column and each of $n$ symbols occurs exactly $k$ times. A transversal of a latin square corresponds to the case $k=1$. For $k>n/4$ we prove that not all $k$-plexes are completable to latin squares. Certain latin squares, including the Cayley tables of many groups, are shown to contain no $(2c+1)$-plex for any integer $c$. However, Cayley tables of soluble groups have a $2c$-plex for each possible $c$. We conjecture that this is true for all latin squares and confirm this for orders $n\leq8$. Finally, we demonstrate the existence of indivisible $k$-plexes, meaning that they contain no $c$-plex for $1\leq c

A Concise Proof of the Littlewood-Richardson Rule

Abstract: We give a short proof of the Littlewood-Richardson rule using a sign-reversing involution.

On Packing Densities of Permutations

Abstract: The density of a permutation pattern $\pi$ in a permutation $\sigma$ is the proportion of subsequences of $\sigma$ of length $|\pi|$ that are isomorphic to $\pi$. The maximal value of the density is found for several patterns $\pi$, and asymptotic upper and lower bounds for the maximal density are found in several other cases. The results are generalised to sets of patterns and the maximum density is found for all sets of length $3$ patterns.

Words Restricted by Patterns with at Most 2 Distinct Letters

Abstract: We find generating functions for the number of words avoiding certain patterns or sets of patterns with at most 2 distinct letters and determine which of them are equally avoided. We also find exact numbers of words avoiding certain patterns and provide bijective proofs for the resulting formulae.

Combinatorial Laplacian of the Matching Complex

Abstract: A striking result of Bouc gives the decomposition of the representation of the symmetric group on the homology of the matching complex into irreducibles that are self-conjugate. We show how the combinatorial Laplacian can be used to give an elegant proof of this result. We also show that the spectrum of the Laplacian is integral.

Ternary Constant Weight Codes

Abstract: Let $A_3(n,d,w)$ denote the maximum cardinality of a ternary code with length $n$, minimum distance $d$, and constant Hamming weight $w$. Methods for proving upper and lower bounds on $A_3(n,d,w)$ are presented, and a table of exact values and bounds in the range $n \leq 10$ is given.

Sum List Coloring 2 n Arrays

Abstract: Ag raph isf-choosable if for every collection of lists with list sizes specied by f there is a proper coloring using colors from the lists. The sum choice number is the minimum over all choosable functions f of the sum of the sizes in f .W e show that the sum choice number of a 2 n array (equivalent to list edge coloring K2;n and to list vertex coloring the cartesian product K22Kn )i sn 2 + d5n=3e.

On the number of distributive lattices

Abstract: We investigate the numbers $d_k$ of all (isomorphism classes of) distributive lattices with $k$ elements, or, equivalently, of (unlabeled) posets with $k$ antichains. Closely related and useful for combinatorial identities and inequalities are the numbers $v_k$ of vertically indecomposable distributive lattices of size $k$. We present the explicit values of the numbers $d_k$ and $v_k$ for $k

Generating a random sink-free orientation in quadratic time

Abstract: A sink-free orientation of a finite undirected graph is a choice of orientation for each edge such that every vertex has out-degree at least 1. Bubley and Dyer (1997) use Markov Chain Monte Carlo to sample approximately from the uniform distribution on sink-free orientations in time $O(m^3 \log (1 / \varepsilon))$, where $m$ is the number of edges and $\varepsilon$ the degree of approximation. Huber (1998) uses coupling from the past to obtain an exact sample in time $O(m^4)$. We present a simple randomized algorithm inspired by Wilson's cycle popping method which obtains an exact sample in mean time at most $O(nm)$, where $n$ is the number of vertices.

Thue-like Sequences and Rainbow Arithmetic Progressions

Abstract: A sequence $u=u_{1}u_{2}...u_{n}$ is said to be nonrepetitive if no two adjacent blocks of $u$ are exactly the same. For instance, the sequence $a{\bf bcbc}ba$ contains a repetition $bcbc$, while $abcacbabcbac$ is nonrepetitive. A well known theorem of Thue asserts that there are arbitrarily long nonrepetitive sequences over the set $\{a,b,c\}$. This fact implies, via Konig's Infinity Lemma, the existence of an infinite ternary sequence without repetitions of any length. In this paper we consider a stronger property defined as follows. Let $k\geq 2$ be a fixed integer and let $C$ denote a set of colors (or symbols). A coloring $f:{\bf N}\rightarrow C$ of positive integers is said to be $k$ -nonrepetitive if for every $r\geq 1$ each segment of $kr$ consecutive numbers contains a $k$-term rainbow arithmetic progression of difference $r$. In particular, among any $k$ consecutive blocks of the sequence $f=f(1)f(2)f(3)...$ no two are identical. By an application of the Lovasz Local Lemma we show that the minimum number of colors in a $k$-nonrepetitive coloring is at most $2^{-1}e^{k(2k-1)/(k-1)^{2}}k^{2}(k-1)+1$. Clearly at least $k+1$ colors are needed but whether $O(k)$ suffices remains open. This and other types of nonrepetitiveness can be studied on other structures like graphs, lattices, Euclidean spaces, etc., as well. Unlike for the classical Thue sequences, in most of these situations non-constructive arguments seem to be unavoidable. A few of a range of open problems appearing in this area are presented at the end of the paper.

Relaxed Graceful Labellings of Trees

Abstract: A graph $G$ on $m$ edges is considered graceful if there is a labelling $f$ of the vertices of $G$ with distinct integers in the set $\{0,1,\dots,m\}$ such that the induced edge labelling $g$ defined by $g(uv)=|f(u)-f(v)|$ is a bijection to $\{1,\dots,m\}$. We here consider some relaxations of these conditions as applied to tree labellings: 1. Edge-relaxed graceful labellings, in which repeated edge labels are allowed, 2. Range-relaxed graceful labellings, in which the upper bound $m'$ is allowed to go higher than the number of edges, and 3. Vertex-relaxed graceful labellings, in which repeated vertex labels are allowed. The first of these had been looked at by Rosa and Siraň (1995). Here some linear bounds in the relevant metrics are given for range-relaxed and vertex-relaxed graceful labellings.

MacWilliams identities and matroid polynomials

Abstract: We present generalisations of several MacWilliams type identities, including those by Klove and Shiromoto, and of the theorems of Greene and Barg that describe how the Tutte polynomial of the vector matroid of a linear code determines the $r$th support weight enumerators of the code. One of our main tools is a generalisation of a decomposition theorem due to Brylawski.

Non-Repetitive Tilings

Abstract: In 1906 Axel Thue showed how to construct an infinite non-repetitive (or square-free) word on an alphabet of size 3. Since then this result has been rediscovered many times and extended in many ways. We present a two-dimensional version of this result. We show how to construct a rectangular tiling of the plane using 5 symbols which has the property that lines of tiles which are horizontal, vertical or have slope +1 or $-1$ contain no repetitions. As part of the construction we introduce a new type of word, one that is non-repetitive up to mod k , which is of interest in itself. We also indicate how our results might be extended to higher dimensions.

The packing density of other layered permutations

Abstract: In this paper the packing density of various layered permutations is calculated, thus solving some problems suggested by Albert, Atkinson, Handley, Holton $\&$ Stromquist [Electron. J. Combin. 9 (2002), $\#$R5]. Specifically, the density is found for layered permutations of type $[m_1, \ldots, m_r]$ when $\log(r+1)\le \min\{ m_i\}$. It is also shown how to derive good estimates for the packing density of permutations of type $[k,1,k]$ when $k\ge 3$. Both results are based on establishing the number of layers in near optimal permutations using a layer-merging technique.

Cycle index, weight enumerator, and Tutte polynomial

Abstract: With every linear code is associated a permutation group whose cycle index is the weight enumerator of the code (up to a trivial normalisation). There is a class of permutation groups (the IBIS groups ) which includes the groups obtained from codes as above. With every IBIS group is associated a matroid; in the case of a group from a code, the matroid differs only trivially from that which arises directly from the code. In this case, the Tutte polynomial of the code specialises to the weight enumerator (by Greene's Theorem), and hence also to the cycle index. However, in another subclass of IBIS groups, the base-transitive groups , the Tutte polynomial can be derived from the cycle index but not vice versa . I propose a polynomial for IBIS groups which generalises both Tutte polynomial and cycle index.

A Prolific Construction of Strongly Regular Graphs with the $n$-e.c. Property

Abstract: A graph is $n$-e.c.$\,$ ($n$-existentially closed) if for every pair of subsets $U$, $W$ of the vertex set $V$ of the graph such that $U\cap W=\emptyset$ and $|U|+|W|=n$, there is a vertex $v\in V-(U\cup W)$ such that all edges between $v$ and $U$ are present and no edges between $v$ and $W$ are present. A graph is strongly regular if it is a regular graph such that the number of vertices mutually adjacent to a pair of vertices $v_1,v_2\in V$ depends only on whether or not $\{v_1,v_2\}$ is an edge in the graph. The only strongly regular graphs that are known to be $n$-e.c. for large $n$ are the Paley graphs. Recently D. G. Fon-Der-Flaass has found prolific constructions of strongly regular graphs using affine designs. He notes that some of these constructions were also studied by Wallis. By taking the affine designs to be Hadamard designs obtained from Paley tournaments, we use probabilistic methods to show that many non-isomorphic strongly regular $n$-e.c. graphs of order $(q+1)^2$ exist whenever $q\geq 16 n^2 2^{2n}$ is a prime power such that $q\equiv 3\!\!\!\pmod{4}$.

Spanning Trees and Function Classes

Abstract: If $G=K_n$ is the complete graph, the classical Pruffer correspondence gives a natural bijection between all spanning trees of $G$ (i.e., all Cayley trees) and all functions from a set of $n-2$ elements to a set of $n$ elements. If $G$ is a complete multipartite graph, then such bijections have been studied by Egecioglu and Remmel. In this paper, we define a class of directed graphs, called filtered digraphs, and describe a natural class of bijections between oriented spanning forests of these digraphs and associated classes of functions. We derive multivariate generating functions for the oriented spanning forests which arise in this context, and we link basic properties of these spanning forests to properties of the functions to which they correspond. This approach yields a number of new results for directed graphs. Moreover, in the undirected case, various specializations of our multivariate generating function not only include various known results but also give a number of new results.

On a Theorem of Erdős, Rubin, and Taylor on Choosability of Complete Bipartite Graphs

Abstract: Erdős, Rubin, and Taylor found a nice correspondence between the minimum order of a complete bipartite graph that is not $r$-choosable and the minimum number of edges in an $r$-uniform hypergraph that is not $2$-colorable (in the ordinary sense). In this note we use their ideas to derive similar correspondences for complete $k$-partite graphs and complete $k$-uniform $k$-partite hypergraphs.

The minimum number of monotone subsequences

Abstract: Erd˝ os and Szekeres showed that any permutation of length n k 2 +1 contains a monotone subsequence of length k + 1. A simple example shows that there need be no more than (n mod k) dn/ke k+1 +( k (n mod k)) bn/kc k+1 such subsequences; we conjecture that this is the minimum number of such subsequences. We prove this for k = 2, with a complete characterisation of the extremal permutations. For k> 2a ndn k(2k 1), we characterise the permutations containing the minimum number of monotone subsequences of length k + 1 subject to the additional constraint that all such subsequences go in the same direction (all ascending or all descending); we show that there are 2 k n mod k C 2k 2 k such extremal permutations, where Ck = 1 k+1 2k k is the k th Catalan number. We conjecture, with some supporting computational evidence, that permutations with a minimum number of monotone (k + 1)-subsequences must have all such subsequences in the same direction if n k(2k 1), except for the case of k =3 andn = 16.

Graph Color Extensions: When Hadwiger's Conjecture and Embeddings Help

Abstract: Suppose $G$ is $r$-colorable and $P \subseteq V(G)$ is such that the components of $G[P]$ are far apart. We show that any $(r+s)$-coloring of $G[P]$ in which each component is $s$-colored extends to an $(r+s)$-coloring of $G$. If $G$ does not contract to $K_5$ or is planar and $s \geq 2$, then any $(r+s-1)$-coloring of $P$ in which each component is $s$-colored extends to an $(r+s-1)$-coloring of $G$. This result uses the Four Color Theorem and its equivalence to Hadwiger's Conjecture for $k = 5$. For $s=2$ this provides an affirmative answer to a question of Thomassen. Similar results hold for coloring arbitrary graphs embedded in both orientable and non-orientable surfaces.

Exchange Symmetries in Motzkin Path and Bargraph models of Copolymer Adsorption

Abstract: In a previous work [26], by considering paths that are partially weighted, the generating function of Dyck paths was shown to possess a type of symmetry, called an exchange relation , derived from the exchange of a portion of the path between weighted and unweighted halves. This relation is particularly useful in solving for the generating functions of certain models of vertex-coloured Dyck paths; this is a directed model of copolymer adsorption, and in a particular case it is possible to find an asymptotic expression for the adsorption critical point of the model as a function of the colouring. In this paper we examine Motzkin path and partially directed walk models of the same adsorbing directed copolymer problem. These problems are an interesting generalisation of previous results since the colouring can be of either the edges, or the vertices, of the paths. We are able to find asymptotic expressions for the adsorption critical point in the Motzkin path model for both edge and vertex colourings, and for the partially directed walk only for edge colourings. The vertex colouring problem in partially directed walks seems to be beyond the scope of the methods of this paper, and remains an open question. In both these cases we first find exchange relations for the generating functions, and use those to find the asymptotic expression for the adsorption critical point.

Compositions of Random Functions on a Finite Set

Abstract: If we compose sufficiently many random functions on a finite set, then the composite function will be constant. We determine the number of compositions that are needed, on average. Choose random functions $f_1, f_2,f_3,\dots $ independently and uniformly from among the $n^n$ functions from $[n]$ into $[n]$. For $t>1$, let $g_t=f_t\circ f_{t-1}\circ \cdots \circ f_1$ be the composition of the first $t$ functions. Let $T$ be the smallest $t$ for which $g_t$ is constant(i.e. $g_t(i)=g_t(j)$ for all $i,j$). We prove that $E(T)\sim 2n$ as $n\rightarrow\infty$, where $E(T)$ denotes the expected value of $T$.

Toida's Conjecture is True

Abstract: Let $S$ be a subset of the units in ${\bf Z_n}$. Let ${\Gamma}$ be a circulant graph of order $n$ (a Cayley graph of ${\bf Z_n}$) such that if $ij\in E({\Gamma})$, then $i - j$ (mod $n$) $\in S$. Toida conjectured that if $\Gamma'$ is another circulant graph of order $n$, then ${\Gamma}$ and ${\Gamma '}$ are isomorphic if and only if they are isomorphic by a group automorphism of ${\bf Z_n}$ In this paper, we prove that Toida's conjecture is true. We further prove that Toida's conjecture implies Zibin's conjecture, a generalization of Toida's conjecture.

The Directed Anti-Oberwolfach Solution: Pancyclic 2-Factorizations of Complete Directed Graphs of Odd Order

Abstract: The directed anti-Oberwolfach problem asks for a 2-factorization (each factor has in-degree 1 and out-degree 1 for a total degree of two) of $K_{2n+1}$, not with consistent cycle components in each 2-factor like the Oberwolfach problem, but such that every admissible cycle size appears at least once in some 2-factor. The solution takes advantage of both Piotrowski's decomposition techniques used to solve Oberwolfach problems and the techniques used by the author to solve the undirected anti-Oberwolfach problem.

Computation in Coxeter groups I. Multiplication

Abstract: An efficient and purely combinatorial algorithm for calculating products in arbitrary Coxeter groups is presented, which combines ideas of Fokko du Cloux and myself. Proofs are largely based on geometry. The algorithm has been implemented in practical Java programs, and runs surprisingly quickly. It seems to be good enough in many interesting cases to build the minimal root reflection table of Brink and Howlett, which can be used for a more efficient multiplication routine.

Minimum connected dominating sets of random cubic graphs

Abstract: We present a simple heuristic for finding a small connected dominating set of cubic graphs. The average-case performance of this heuristic, which is a randomised greedy algorithm, is analysed on random $n$-vertex cubic graphs using differential equations. In this way, we prove that the expected size of the connected dominating set returned by the algorithm is asymptotically almost surely less than $0.5854n$.