Commutative combinatorial Hopf algebras

We propose several constructions of commutative or cocommutative Hopf algebras based on various combinatorial structures and investigate the relations between them. A commutative Hopf algebra of permutations is obtained by a general construction based on graphs, and its noncommutative dual is realized in three different ways, in particular, as the Grossman–Larson algebra of heap-ordered trees. Extensions to endofunctions, parking functions, set compositions, set partitions, planar binary trees, and rooted forests are discussed. Finally, we introduce one-parameter families interpolating between different structures constructed on the same combinatorial objects.     

Exact Enumeration of 1342-Avoiding Permutations: A Close Link with Labeled Trees and Planar Maps

Solving the first nonmonotonic, longer-than-three instance of a classic enumeration problem, we obtain the generating functionH(x) of all 1342-avoiding permutations of lengthnas well as anexactformula for their numberS_n(1342). While achieving this, we bijectively prove that the number of indecomposable 1342-avoiding permutations of lengthnequals that of labeled plane trees of a certain type onnvertices recently enumerated by Cori, Jacquard, and Schaeffer, which is in turn known to be equal to the number of rooted bicubic maps enumerated by Tutte (Can. J. Math.33(1963), 249–271). Moreover,H(x) turns out to be algebraic, proving the first nonmonotonic, longer-than-three instance of a conjecture of Noonan and Zeilberger (Adv. Appl. Math.17(1996), 381–407). We also prove thatconverges to 8, so in particular, lim_n→∞(S_n(1342)/S_n(1234))=0.     

Coloring rules for finite trees,and probabilities of monadic second order sentences

A system of coloring rules is a set of rules for coloring the vertices of any finite rooted tree, starting at its leaves, which has the property that the color assigned to each vertex depends only on how many of its immediate predecessors there are of each color. The asymptotic behavior of the fraction of n vertex trees with a given root color is investigated. As a primary application, if φ is any monadic second-order sentence, then the fractions μ_n(φ), ν_n(φ) of labeled and unlabeled rooted trees satisfying φ, converge to limiting probabilities. An extension of the idea shows that for a particular model of Boolean formulas in k variables, k fixed, the fraction of such formulas of size n which compute a given Boolean function approaches a positive limit as n→∞. © 1997 John Wiley & Sons, Inc. Random Struct. Alg., 10 , 453–485, 1997     

Mappings of acyclic and parking functions

The two functions in question are mappings: [n]→[n], with [n] = {1, 2,?,n}. The acyclic function may be represented by forests of labeled rooted trees, or by free trees withn + 1 points; the parking functions are associated with the simplest ballot problem. The total number of each is (n + 1) ^n-1. The first of two mappings given is based on a simple mapping, due to H. O. Pollak, of parking functions on tree codes. In the second, each kind of function is mapped on permutations, arising naturally from characterizations of the functions. Several enumerations are given to indicate uses of the mappings.     

Forests of label-increasing trees

Label-increasing trees are fully labeled rooted trees with the restriction that the labels are in increasing order on every path from the root; the best known example is the binary case—no tree with more than two branches at the root, or internal vertices of degree greater than three—extensively examined by Foata and Schutzenberger in A Survey of Combinatorial Theory. The forests without branching restrictions are enumerated by number of trees by F_n(x) = x(x + 1)…(x + n ? 1), n >1 (F₀(x) = 1), whose equivalent: F_n(x) = Y_n(xT₁,…, xT_n), F_n(1)= T_{n + 1} = n!, is readily adapted to branching restriction.     

Les hypercartes planaires sont des arbres tres bien etiquetes

R. Cori and B. Vauquelin have constructed (cf[1]) a one to one correspondence from rooted planar maps onto rooted well-labeled trees (trees whose vertices are labeled with natural numbers that differ by at most one on adjacent vertices). This correspondence does not associate other families of planar maps (e.g. planar hypermaps,...) and easily definable families of trees. The main result of this paper (Theorem 1, Section II) is to construct a new one to one correspondence from rooted planar maps onto rooted well-labeled trees which also associates rooted planar hypermaps with n edge-ends (called ‘brin’ in French) and rooted very well-labeled trees (well labeled trees whose adjacent vertices have not the same label) with n edges. This last result is given in Section 3, Theorem 2.The coding of rooting very well-labeled trees by words extending Dyck's words (or parenthesis systems), allows their enumeration, hence the enumeration of rooted planar hypermaps. This side is the subject of a work in progress under B. Vauquelin.     

On avoidable and unavoidable trees

A directed tree is a rooted tree if there is one vertex (the root) of in-degree 0 and every other vertex has in-degree 1. The depth of a rooted tree is the length of a longest path from the root. A directed graph G is called n-unavoidable if every tournament of order n contains it as a subgraph. M. Saks and V. Sós [“On Unavoidable Subgraphs of Tournaments,” Colloquia Mathematica Societatis Janos Bolyai 37, Finite and Infinite Sets, Eger, Hungary (1981), 663–674] constructed unavoidable rooted spanning trees of depth 3. There they wrote, “It is natural to ask how small the depth of a spanning n-unavoidable rooted tree can be.” In this paper we construct unavoidable rooted spanning trees of depth 2. Note that the depth 2 is the best we can do. For each n define λ(n) to be the largest real number such that every claw with degree d ≤ λ(n)n is n-unavoidable. The example in X. Lu [“On Claws Belonging to Every Tournament,” Combinatorica, Vol. 11 (1991), pp. 173–179] showed that λ(n) < 1/2 for sufficiently large n, but the upper bound on λ(n) given there tends to 1/2 for large n. Let λ be the lim sup of λ(n) as n tends to infinity. In this paper we show that λ is strictly less than 1/2, specifically λ ≤ 25/52. © 1996 John Wiley & Sons, Inc.     

Explicit Formulae for Some Kazhdan-Lusztig Polynomials

We consider the Kazhdan-Lusztig polynomials P _u,v (q) indexed by permutations u, v having particular forms with regard to their monotonicity patterns. The main results are the following. First we obtain a simplified recurrence relation satisfied by P _u,v (q) when the maximum value of v S_n occurs in position n – 2 or n – 1. As a corollary we obtain the explicit expression for P_{e,3 4 ... n 1 2}(q) (where e denotes the identity permutation), as a q-analogue of the Fibonacci number. This establishes a conjecture due to M. Haiman. Second, we obtain an explicit expression for P_{e, 3 4 ... (n – 2) n (n – 1) 1 2}(q). Our proofs rely on the recurrence relation satisfied by the Kazhdan-Lusztig polynomials when the indexing permutations are of the form under consideration, and on the fact that these classes of permutations lend themselves to the use of induction. We present several conjectures regarding the expression for P _u,v (q) under hypotheses similar to those of the main results.     

Tutte polynomials for trees

We define two two-variable polynomials for rooted trees and one two-variable polynomial for unrooted trees, all of which are based on the coranknullity formulation of the Tutte polynomial of a graph or matroid. For the rooted polynomials, we show that the polynomial completely determines the rooted tree, i.e., rooted trees T₁ and T₂ are isomorphic if and only if f(T₁) = f(T₂). The corresponding question is open in the unrooted case, although we can reconstruct the degree sequence, number of subtrees of size k for all k, and the number of paths of length k for all k from the (unrooted) polynomial. The key difference between these three polynomials and the standard Tutte polynomial is the rank function used; we use pruning and branching ranks to define the polynomials. We also give a subtree expansion of the polynomials and a deletion-contraction recursion they satisfy.     

Conjugacy in Permutation Representations of the Symmetric Group

Although the conjugacy classes of the general linear group are known, it is not obvious (from the canonic form of matrices) that two permutation matrices are similar if and only if they are conjugate as permutations in the symmetric group, i.e., that conjugacy classes of S _n do not unite under the natural representation. We prove this fact, and give its application to the enumeration of fixed points under a natural action of S _n  × S _n . We also consider the permutation representations of S _n which arise from the action of S _n on ordered tuples and on unordered subsets, and classify which of them unite conjugacy classes and which do not.     

Combinatorial interpretations for TG(1, −1)

Let G be a connected and simple graph with vertex set {1, 2, …, n + 1} and T_G(x, y) the Tutte polynomial of G. In this paper, we give combinatorial interpretations for T_G(1, ?1). In particular, we give the definitions of even spanning tree and left spanning tree. We prove T_G(1, ?1) is the number of even‐left spanning trees of G. We associate a permutation with a spanning forest of G and give the definition of odd G‐permutations. We show T_G(1, ?1) is the number of odd G‐permutations. We give a bijection from the set of odd K_{n + 1}‐permutations to the set of alternating permutations on the set {1, 2, …, n}. Copyright © 2011 Wiley Periodicals, Inc. J Graph Theory 69: 341–348, 2012     

The cubic mapping graph for the ring of Gaussian integers modulo <Emphasis Type="Italic">n</Emphasis>

The article studies the cubic mapping graph Γ(n) of ℤ _n [i], the ring of Gaussian integers modulo n. For each positive integer n > 1, the number of fixed points and the in-degree of the elements [`1]\overline 1 and [`0]\overline 0 in Γ(n) are found. Moreover, complete characterizations in terms of n are given in which Γ₂(n) is semiregular, where Γ₂(n) is induced by all the zero-divisors of ℤ _n [i].     

Transitivity And Connectivity Of Permutations

It was observed for years, in particular in quantum physics, that the number of connected permutations of [0; n] (also called indecomposable permutations), i. e. those such that for any i < n there exists j > i with (j) < i, equals the number of pointed hypermaps of size n, i. e. the number of transitive pairs (, ) of permutations of a set of cardinality n with a distinguished element.The paper establishes a natural bijection between the two families. An encoding of maps follows. To Chantal, that we may stay connected beyond the simple line of time.     

Packing and covering constants for certain families of trees. I

If ? denotes a family of rooted trees, let p_k(n) and c_k(n) denote the average value of the k-packing and k-covering numbers of trees in ? that have n nodes. We assume, among other things, that the generating function y of trees in ? satisfies a relation of the type y = x?(y) for some power series ?. We show that the limits of p_k(n)/n and c_k(n)/n as n → ∞ exist and we describe how to evaluate these limits.     

On the error of quadrature formulae for Cauchy principal value integrals based on piecewise interpolation

We consider the computation of the Cauchy principal value integral by quadrature formulae Q _n ^F [f] of compound type, which are obtained by replacing f by a piecewise defined function F_n[f]. The behaviour of the constants k_{i, n} in the estimates [R _n ^F [f]] |⩽K _i,n ‖f ⁽ⁱ⁾‖_∞ (where R _n ^F [f] is the quadrature error) is determined for fixed i and n→∞, which means that not only the order, but also the coefficient of the main term of k_{i, n} is determined. The behaviour of these error constants k_{i, n} is compared with the corresponding ones obtained for the method of subtraction of the singularity. As it turns out, these error constants have, in general, the same asymptotic behaviour.     

Achiral plane trees

Harary and Robinson showed that the number a_n of achiral planted plane trees with n points coincides with the number p_n of achiral plane trees with n points, for n ? 2. They posed the problem of finding a natural structural correspondence which explains this coincidence. In the present paper this problem is answered by constructing two-to-one correspondences from certain sets of binary sequences to each of the sets of trees concerned, giving a structural basis for the equation 2a_n = 2p_n. Answers are also supplied to similar correspondence-type problems of Harary and Robinson, concerning planted plane trees, and achiral rooted plane trees. In addition, each of these four types of plane trees are counted with numbers of points and endpoints as the enumeration parameters. The results all show a symmetry with respect to the number of endpoints which is not shared by the set of all plane trees.     

Construction of a local and global Lyapunov function for discrete dynamical systems using radial basis functions

The basin of attraction of an asymptotically stable fixed point of the discrete dynamical system given by the iteration x_n+1=g(x_n) can be determined through sublevel sets of a Lyapunov function. In Giesl [On the determination of the basin of attraction of discrete dynamical systems. J. Difference Equ. Appl. 13(6) (2007) 523–546] a Lyapunov function is constructed by approximating the solution of a difference equation using radial basis functions. However, the resulting Lyapunov function is non-local, i.e. it has no negative discrete orbital derivative in a neighborhood of the fixed point. In this paper we modify the construction method by using the Taylor polynomial and thus obtain a Lyapunov function with negative discrete orbital derivative both locally and globally.     

The Height of Increasing Trees

Increasing trees have been introduced by Bergeron, Flajolet, and Salvy [1]. This kind of notion covers several well-know classes of random trees like binary search trees, recursive trees, and plane oriented (or heap ordered) trees. We consider the height of increasing trees and prove for several classes of trees (including the above mentioned ones) that the height satisfies EH _n ~ γlogn (for some constant γ > 0) and Var H _n =  O(1) as n → ∞. The methods used are based on generating functions. This research was supported by the Austrian Science Foundation FWF, project S9604, that is part of the Austrian National Research Network "Analytic Combinatorics and Probabilistic Number Theory".     

Tail bounds for the height and width of a random tree with a given degree sequence

Fix a sequence c = (c₁,…,c_n) of non‐negative integers with sum n ? 1. We say a rooted tree T has child sequence c if it is possible to order the nodes of T as v₁,…,v_n so that for each 1 ≤ i ≤ n, v_i has exactly c_i children. Let ${\mathcal T}Fix a sequence c = (c₁,…,c_n) of non‐negative integers with sum n ? 1. We say a rooted tree T has child sequence c if it is possible to order the nodes of T as v₁,…,v_n so that for each 1 ≤ i ≤ n, v_i has exactly c_i children. Let ${\mathcal T}$ be a plane tree drawn uniformly at random from among all plane trees with child sequence c . In this note we prove sub‐Gaussian tail bounds on the height (greatest depth of any node) and width (greatest number of nodes at any single depth) of ${\mathcal T}$. These bounds are optimal up to the constant in the exponent when c satisfies $\sum_{i=1}^n c_i^2=O(n)$; the latter can be viewed as a “finite variance” condition for the child sequence. © 2012 Wiley Periodicals, Inc. Random Struct. Alg., 2012