Ordering trees by the Laplacian coefficients

Motivated by a Mohar’s paper proposing “how to order trees by the Laplacian coefficients”, we investigate a partial ordering of trees with diameters 3 and 4 by the Laplacian coefficients. These results are used to determine several orderings of trees by the Laplacian coefficients.     

Ordering trees by algebraic connectivity

LetG be a graph onn vertices. Denote byL(G) the difference between the diagonal matrix of vertex degrees and the adjacency matrix. It is not hard to see thatL(G) is positive semidefinite symmetric and that its second smallest eigenvalue,a(G) > 0, if and only ifG is connected. This observation led M. Fiedler to calla(G) thealgebraic connectivity ofG. Given two trees,T ₁ andT ₂, the authors explore a graph theoretic interpretation for the difference betweena(T ₁) anda(T ₂).Research supported by ONR contract 85K0335     

Ordering trees by their spectral radii

Let T be a tree with n vertices and let A（T） be the adjacency matrix of T. Spectral radius of T is the largest eigenvalue of A（T）. Wu et al. [Wu, B.F., Yuan, X.Y, and Xiao, E.L. On the spectral radii of trees, Journal of East China Normal University （Natural Science）, 3：22-28 （2004）] determined the first seven trees of order n with the smallest spectral radius. In this paper, we extend this ordering by determining the trees with the eighth to the tenth smallest spectral radius among all trees with n vertices.     

Ordering of trees with fixed matching number by the Laplacian coefficients

Let G be a simple undirected graph with the characteristic polynomial of its Laplacian matrix L(G), . Aleksandar Ili? [A. Ili?, Trees with minimal Laplacian coefficients, Comput. Math. Appl. 59 (2010) 2776-2783] identified n-vertex trees with given matching number q which simultaneously minimize all Laplacian coefficients. In this paper, we give another proof of this result. Generalizing the approach in the above paper, we determine n-vertex trees with given matching number q which have the second minimal Laplacian coefficients. We also identify the n-vertex trees with a perfect matching having the largest and the second largest Laplacian coefficients, respectively. Extremal values on some indices, such as Wiener index, modified hyper-Wiener index, Laplacian-like energy, incidence energy, of n-vertex trees with matching number q are obtained in this paper.     

Recurrence relations with two indices and even trees

Recently, the author, Mansour, introduced a combinatorial problem, called Hobby's problem, to study different types of recurrence relations with two indices. Moreover, he presented several recurrence relations with two indices related to Dyck paths and Schröder paths. In this paper, we generalize Hobby's problem to study other types of recurrence relations with two indices for which a combinatorial method provides a complete solution. Combinatorially, we describe these recurrence relations as a set of lattice paths in the second octant of the plane integer lattice, and then we map bijectively these lattice paths to the set of even trees. Analytically, we use the kernel method technique to solve these recurrence relations.     

树的Nordhaus-Gaddum类型谱半径的排序

给出了n阶树的Nordhaus-Gaddum类型谱半径即图及其补图的谱半径之和的可达上界:ρ(T) ρ(Tc)≤■ n-2,等号成立当且仅当T K1,n-1,其中Tc为T的补图,K1,n-1为n阶星图.同时证明了对于n阶双星图S(a,b)的Nordhaus-Gaddum类型谱半径随a的值单调上升,其中[n-1/2]≤a≤n-3.     

Wiener indices of balanced binary trees

We study a new family of trees for computation of the Wiener indices. We introduce general tree transformations and derive formulas for computing the Wiener indices when a tree is modified. We present several algorithms to explore the Wiener indices of our family of trees. The experiments support new conjectures about the Wiener indices.     

Asymptotics for the wiener sausage with drift

Summary A particle is considered which moves in ^d according to a Brownian motion with drifth0. The space is assumed to contain random traps. The probability of survival of the particle up to timeT decays exponentially asT with a positive decay rate . is shown to be a non-analytic function of |h|. For small |h| the decay rate is given by (h)=1/2|h|²; but if |h| exceeds a certain critical value, (h) depends also on the parameters describing trapping. Upper and lower bounds for (h) are given, which imply the asymptotic linearity of (h) for large |h|. The critical point marks a transition from localized to delocalized behavior. A variational formula for the decay rate is given on the level of generalized processes, which elucidates the mathematical mechanism behind observations made earlier by Grassberger and Procaccia on the basis of computer simulations.Supported by Deutsche Forschungsgemeinschaft     

Acyclic chromatic indices of planar graphs with large girth

An acyclic edge coloring of a graph G is a proper edge coloring such that no bichromatic cycles are produced. The acyclic chromatic index a^′(G) of G is the smallest k such that G has an acyclic edge coloring using k colors.In this paper, we prove that every planar graph G with girth g(G) and maximum degree Δ has a^′(G)=Δ if there exists a pair (k,m)∈{(3,11),(4,8),(5,7),(8,6)} such that G satisfies Δ≥k and g(G)≥m.     

On the Ramsey number of trees versus graphs with large clique number

Chvátal established that r(T_m, K_n) = (m – 1)(n – 1) + 1, where T_m is an arbitrary tree of order m and K_n is the complete graph of order n. This result was extended by Chartrand, Gould, and Polimeni who showed K_n could be replaced by a graph with clique number n and order n + 1 provided n ≧ 3 and m ≧ 3. We further extend these results to show that K_n can be replaced by any graph on n + 2 vertices with clique number n, provided n ≧ 5 and m ≧ 4. We then show that further extensions, in particular to graphs on n + 3 vertices with clique number n are impossible. We also investigate the Ramsey number of trees versus complete graphs minus sets of independent edges. We show that r(T_m, K_n –tK₂) = (m – 1)(n – t – 1) + 1 for m ≧ 3, n ≧ 6, where T_m is any tree of order m except the star, and for each t, O ≦ t ≦ [(n – 2)/2].     

Thermodynamic limit for large random trees

We consider Gibbs distributions on finite random plane trees with bounded branching. We show that as the order of the tree grows to infinity, the distribution of any finite neighborhood of the root of the tree converges to a limit. We compute the limiting distribution explicitly and study its properties. We introduce an infinite random tree consistent with these limiting distributions and show that it satisfies a certain form of the Markov property. We also study the growth of this tree and prove several limit theorems including a diffusion approximation. © 2010 Wiley Periodicals, Inc. Random Struct. Alg., 2010     

Integral trees of arbitrarily large diameters

In this paper, we construct trees having only integer eigenvalues with arbitrarily large diameters. In fact, we prove that for every finite set S of positive integers there exists a tree whose positive eigenvalues are exactly the elements of S. If the set S is different from the set {1} then the constructed tree will have diameter 2|S|.     

Properties of a wiener process with coalescence

A Wiener process with coalescence and its analog are discussed. We prove the existence of an initial distribution with preset final probabilities for this analog and investigate the problem of the existence of such distributions concentrated at a single point or absolutely continuous with respect to the Lebesgue measure. The behavior of a semigroup of a Wiener process with coalescence in the two-dimensional case and properties of a Wiener flow with coalescence are studied. __________ Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, No. 4, pp. 489–504, April, 2006.