Non-crossing Trees Research Articles

Consecutive Pattern Avoidances in Non-crossing Trees

In this paper, motivated by the concepts of increasing (alternating) non-crossing trees, the problem of consecutive pattern avoidances in non-crossing trees is proposed. Some given patterns of length two and three are investigated in detail. The Lagrange inversion formula is used to obtain the explicit formulas for these cases. Bijections are established between non-crossing trees avoiding special patterns and Schroder paths.

2-noncrossing trees and 5-ary trees

Recently, Gu et al. [N.S.S. Gu, N.Y. Li, T. Mansour, 2-Binary trees: Bijections and related issues, Discrete Math. 308 (2008) 1209–1221] introduced 2-binary trees and 2-plane trees which are closely related to ternary trees. In this note, we study the 2-noncrossing tree, a noncrossing tree in which each vertex is colored black or white and there is no ascent ( u , v ) such that both the vertices u and v are colored black. By using the representation of Panholzer and Prodinger for noncrossing trees, we find a correspondence between the set of 2-noncrossing trees of n edges with a black root and the set of 5-ary trees with n internal vertices.

String pattern avoidance in generalized non-crossing trees

Combinatorics The problem of string pattern avoidance in generalized non-crossing trees is studied. The generating functions for generalized non-crossing trees avoiding string patterns of length one and two are obtained. The Lagrange inversion formula is used to obtain the explicit formulas for some special cases. A bijection is also established between generalized non-crossing trees with special string pattern avoidance and little Schr ̈oder paths.

Dyck paths with coloured ascents

We introduce a notion of Dyck paths with coloured ascents. For several ways of colouring, when the set of colours is itself some class of lattice paths, we establish bijections between sets of such paths and other combinatorial structures, such as non-crossing trees, dissections of a convex polygon, etc. In some cases enumeration gives new expression for sequences enumerating these structures.

2-Binary trees: Bijections and related issues

A 2- binary tree is a binary rooted tree whose root is colored black and the other vertices are either black or white. We present several bijections concerning different types of 2-binary trees as well as other combinatorial structures such as ternary trees, non-crossing trees, Schröder paths, Motzkin paths and Dyck paths. We also obtain a number of enumeration results with respect to certain statistics.

Noncrossing Trees and Noncrossing Graphs

We give a parity reversing involution on noncrossing trees that leads to a combinatorial interpretation of a formula on noncrossing trees and symmetric ternary trees in answer to a problem proposed by Hough. We use the representation of Panholzer and Prodinger for noncrossing trees and find a correspondence between a class of noncrossing trees, called proper noncrossing trees, and the set of symmetric ternary trees. The second result of this paper is a parity reversing involution on connected noncrossing graphs which leads to a relation between the number of noncrossing trees with $n$ edges and $k$ descents and the number of connected noncrossing graphs with $n+1$ vertices and $m$ edges.

Descents in Noncrossing Trees

The generating function for descents in noncrossing trees is found. A bijection shows combinatorially why the descent generating function with descents set equal to $2$ is the generating function for connected noncrossing graphs.

Non-crossing trees revisited: cutting down and spanning subtrees

Here we consider two parameters for random non-crossing trees: $\textit{(i)}$ the number of random cuts to destroy a size-$n$ non-crossing tree and $\textit{(ii)}$ the spanning subtree-size of $p$ randomly chosen nodes in a size-$n$ non-crossing tree. For both quantities, we are able to characterise for $n → ∞$ the limiting distributions. Non-crossing trees are almost conditioned Galton-Watson trees, and it has been already shown, that the contour and other usually associated discrete excursions converge, suitable normalised, to the Brownian excursion. We can interpret parameter $\textit{(ii)}$ as a functional of a conditioned random walk, and although we do not have such an interpretation for parameter $\textit{(i)}$, we obtain here limiting distributions, that are also arising as limits of some functionals of conditioned random walks.

Diagonally convex directed polyominoes and even trees: a bijection and related issues

We present a simple bijection between diagonally convex directed (DCD) polyominoes with n diagonals and plane trees with 2 n edges in which every vertex has even degree (even trees), which specializes to a bijection between parallelogram polyominoes and full binary trees. Next we consider a natural definition of symmetry for DCD-polyominoes, even trees, ternary trees, and non-crossing trees, and show that the number of symmetric objects of a given size is the same in all four cases.

Bijections for ternary trees and non-crossing trees

The number N n of non-crossing trees of size n satisfies N n+1=T n where T n enumerates ternary trees of size n. We construct a new bijection to establish that fact. Since T n=(1/(2n+1))( 3n n ) , it follows that 3(3n−1)(3n−2)T n−1=2n(2n+1)T n . We construct two bijections “explaining” this recursion; one of them easily extends to the case of t-ary trees.

Noncrossing trees are almost conditioned Galton–Watson trees

AbstractA noncrossing tree (NC‐tree) is a tree drawn on the plane having as vertices a set of points on the boundary of a circle, and whose edges are straight line segments that do not cross. In this article, we show that NC‐trees with size n are conditioned Galton–Watson trees. As corollaries, we give the limit law of depth‐first traversal processes and the limit profile of NC‐trees. © 2002 John Wiley & Sons, Inc. Random Struct. Alg., 20, 115–125, 2002

Statistics on non-crossing trees

A non-crossing tree is a tree drawn on the plane having as vertices a set of points on the boundary of a circle, and whose edges are straight line segments and do not cross. Continuing previous research on non-crossing trees, we study several new statistics: number of end-points and boundary edges, number of vertices of a given degree, maximum degree, height, and path-length. In some cases, we obtain closed formulae while in others we deduce asymptotic estimates. Our approach is based on generating functions and on several bijections between NC-trees and various other combinatorial objects.