Ballot tilings and increasing trees

We study enumerations of Dyck and ballot tilings, which are tilings of a region determined by two Dyck or ballot paths. We give bijective proofs to two formulae of enumerations of Dyck tilings through Hermite histories. We show that one of the formulae is equal to a certain Kazhdan--Lusztig polynomial. For a ballot tiling, we establish formulae which are analogues of formulae for Dyck tilings. Especially, the generating functions have factorized expressions. The key tool is a planted plane tree and its increasing labellings. We also introduce a generalized perfect matching which is bijective to an Hermite history for a ballot tiling. By combining these objects, we obtain various expressions of a generating function of ballot tilings with a fixed lower path.Comment: 53 page

On the vertex-to-edge duality between the Cayley graph and the coset geometry of von Dyck groups

We prove that the Cayley graph and the coset geometry of the von Dyck group $D(a,b,c)$ are linked by a vertex-to-edge duality.Comment: Accepted for publication on Mathematica Slovaca (23-10-2013); 12 pages, 7 figures; completely rewritten replacement of "On the free Burnside group as a factor of von Dyck group: a geometric perspective

Symmetric Dyck tilings, ballot tableaux and tree-like tableaux of shifted shapes

Symmetric Dyck tilings and ballot tilings are certain tilings in the region surrounded by two ballot paths. We study the relations of combinatorial objects which are bijective to symmetric Dyck tilings such as labeled trees, Hermite histories, and perfect matchings. We also introduce two operations on labeled trees for symmetric Dyck tilings: symmetric Dyck tiling strip (symDTS) and symmetric Dyck tiling ribbon (symDTR). We give two definitions of Hermite histories for symmetric Dyck tilings, and show that they are equivalent by use of the correspondence between symDTS operation and an Hermite history. Since ballot tilings form a subset in the set of symmetric Dyck tilings, we construct an inclusive map from labeled trees for ballot tilings to labeled trees for symmetric Dyck tilings. By this inclusive map, the results for symmetric Dyck tilings can be applied to those of ballot tilings. We introduce and study the notions of ballot tableaux and tree-like tableaux of shifted shapes, which are generalizations of Dyck tableaux and tree-like tableaux, respectively. The correspondence between ballot tableaux and tree-like tableaux of shifted shapes is given by using the symDTR operation and the structure of labeled trees for symmetric Dyck tilings.Comment: 60 page

Quantum Knizhnik-Zamolodchikov equation: reflecting boundary conditions and combinatorics

We consider the level 1 solution of quantum Knizhnik-Zamolodchikov equation with reflecting boundary conditions which is relevant to the Temperley--Lieb model of loops on a strip. By use of integral formulae we prove conjectures relating it to the weighted enumeration of Cyclically Symmetric Transpose Complement Plane Partitions and related combinatorial objects

Enumeration of colored Dyck paths via partial Bell polynomials

We consider a class of lattice paths with certain restrictions on their ascents and down steps and use them as building blocks to construct various families of Dyck paths. We let every building block $P_j$ take on $c_j$ colors and count all of the resulting colored Dyck paths of a given semilength. Our approach is to prove a recurrence relation of convolution type, which yields a representation in terms of partial Bell polynomials that simplifies the handling of different colorings. This allows us to recover multiple known formulas for Dyck paths and related lattice paths in an unified manner.Comment: 10 pages. Submitted for publicatio

Enumerations of humps and peaks in (k,a)(k,a)-paths and (n,m)(n,m)-Dyck paths via bijective proofs

Recently Mansour and Shattuck studied $(k,a)$-paths and gave formulas that relate the total number of humps (peaks) in all $(k,a)$-paths to the number of super $(k,a)$-paths. These results generalize earlier results of Regev on Dyck paths and Motzkin paths. Their proofs are based on generating functions and they asked for bijective proofs for their results. In this paper we first give bijective proofs of Mansour and Shattuck's results, then we extend our study to $(n,m)$-Dyck paths. We give a bijection that relates the total number of peaks in all $(n,m)$-Dyck paths to certain free $(n,m)$-paths when $n$ and $m$ are coprime. From this bijection we get the number of $(n,m)$-Dyck paths with exactly $j$ peaks, which is a generalization of the well-known result that the number Dyck paths of order $n$ with exactly $j$ peaks is the Narayana number $\frac{1}{k}{n-1\choose k-1}{n\choose k-1}$.Comment: 11 page

Deforming the Fredkin spin chain away from its frustration-free point

The Fredkin model describes a spin-half chain segment subject to three-body, correlated-exchange interactions and twisted boundary conditions. The model is frustration-free, and its ground state wave function is known exactly. Its low-energy physics is that of a strong xy ferromagnet with gapless excitations and an unusually large dynamical exponent. We study a generalized spin chain model that includes the Fredkin model as a special tuning point and otherwise interpolates between the conventional ferromagnetic and antiferromagnetic quantum Heisenberg models. We solve for the low-lying states, using exact diagonalization and density-matrix renormalization group calculations, in order to track the properties of the system as it is tuned away from the Fredkin point; we also present exact analytical results that hold right at the Fredkin point. We identify a zero-temperature phase diagram with multiple transitions and unexpected ordered phases. The Fredkin ground state turns out to be particularly brittle, unstable to even infinitesimal antiferromagnetic frustration. We remark on the existence of an "anti-Fredkin" point at which all the contributing spin configurations have a spin structure exactly opposite to those in the Fredkin ground state.Comment: 11 pages, 14 figures; new simulations and analysis of the gap scaling; matches the published versio

Dyck tilings of type DD

We introduce and study cover-inclusive and cover-exclusive Dyck tilings of type $D$. It is shown that the generating functions of Dyck tilings of type $D$ are expressed in terms of the generating function of ballot tilings of type $B$. We introduce link patterns of type $D$ and plane trees for a ballot path, and construct a map from trees to $\mathbb{Z}[q]$. This map gives the generating function of cover-inclusive Dyck tilings of type $D$ associated to the ballot path.Comment: 14 page

Bijections on rr-Shi and rr-Catalan Arrangements

Associated with the $r$-Shi arrangement and $r$-Catalan arrangement in $\Bbb{R}^n$, we introduce a cubic matrix for each region to establish two bijections in a uniform way. Firstly, the positions of minimal positive entries in column slices of the cubic matrix will give a bijection from regions of the $r$-Shi arrangement to $O$-rooted labeled $r$-trees. Secondly, the numbers of positive entries in column slices of the cubic matrix will give a bijection from regions of the $r$-Catalan arrangement to pairings of permutation and $r$-Dyck path. Moreover, the numbers of positive entries in row slices of the cubic matrix will recover the Pak-Stanley labeling, a celebrated bijection from regions of the $r$-Shi arrangement to $r$-parking functions.Comment: 19 pages, 2 figure

Fully Packed Loops in a triangle: matchings, paths and puzzles

Fully Packed Loop configurations in a triangle (TFPLs) first appeared in the study of ordinary Fully Packed Loop configurations (FPLs) on the square grid where they were used to show that the number of FPLs with a given link pattern that has m nested arches is a polynomial function in m. It soon turned out that TFPLs possess a number of other nice properties. For instance, they can be seen as a generalized model of Littlewood-Richardson coefficients. We start our article by introducing oriented versions of TFPLs; their main advantage in comparison with ordinary TFPLs is that they involve only local constraints. Three main contributions are provided. Firstly, we show that the number of ordinary TFPLs can be extracted from a weighted enumeration of oriented TFPLs and thus it suffices to consider the latter. Secondly, we decompose oriented TFPLs into two matchings and use a classical bijection to obtain two families of nonintersecting lattice paths (path tangles). This point of view turns out to be extremely useful for giving easy proofs of previously known conditions on the boundary of TFPLs necessary for them to exist. One example is the inequality d(u)+d(v)<=d(w) where u,v,w are 01-words that encode the boundary conditions of ordinary TFPLs and d(u) is the number of cells in the Ferrers diagram associated with u. In the third part we consider TFPLs with d(w)- d(u)-d(v)=0,1; in the first case their numbers are given by Littlewood-Richardson coefficients, but also in the second case we provide formulas that are in terms of Littlewood-Richardson coefficients. The proofs of these formulas are of a purely combinatorial nature.Comment: 40 pages, 31 figure