The total number of noncrossing partitions of type $Ψ$ is the $n$th Catalan number $\frac{1}{n+1}\binom{2n}{n}$ when $Ψ=A_{n-1}$, and the binomial $\binom{2n}{n}$ when $Ψ=B_n$, and these numbers coincide with the correspondent number of nonnesting partitions. For type A, there are several bijective proofs of this equality, being the intuitive map that locally converts each crossing to a nesting one of them. In this paper we present a bijection between nonnesting and noncrossing partitions of types A and B that generalizes the type A bijection that locally converts each crossing to a nesting.
Set partitions avoiding $k$-crossing and $k$-nesting have been extensively studied from the aspects of both combinatorics and mathematical biology. By using the generating tree technique, the obstinate kernel method and Zeilberger's algorithm, Lin confirmed a conjecture due independently to the author and Martinez-Savage that asserts inversion sequences with no weakly decreasing subsequence of length 3 and enhanced 3-nonnesting partitions have the same cardinality. In this paper, we provide a bijective proof of this conjecture. Our bijection also enables us to provide a new bijective proof of …
Although both noncrossing partitions and nonnesting partitions are uniformly enumerated for Weyl groups, the exact relationship between these two sets of combinatorial objects remains frustratingly mysterious. In this paper, we give a precise combinatorial answer in the case of the symmetric group: for any standard Coxeter element, we construct an equivariant bijection between noncrossing partitions under the Kreweras complement and nonnesting partitions under a Coxeter-theoretically natural cyclic action we call the Kroweras complement. Our equivariant bijection is the unique bijection that i…
For a fixed integer $k$, we consider the set of noncrossing partitions, where both the block sizes and the difference between adjacent elements in a block is $1\bmod k$. We show that these $k$-indivisible noncrossing partitions can be recovered in the setting of subgroups of the symmetric group generated by $(k+1)$-cycles, and that the poset of $k$-indivisible noncrossing partitions under refinement order has many beautiful enumerative and structural properties. We encounter $k$-parking functions and some special Cambrian lattices on the way, and show that a special class of lattice paths cons…
序列x= x_1x_2. x_n $ is said to be an ascent sequence of length $ n $ if it satisfies x_1=0 and $ 0 \leq x_i \leq asc（x_1x_2. x_{i-1}）+1 $ for all $ 2 \leq i \leq n $, where $ asc（x_1x_2. x_{i-1}）$ is the number of ascents in the sequence $ x_1x_2. x_{i-1} $. Recently, Duncan and Steingr\'ımsson proposed the conjecture that 210-avoiding ascent sequences of length $ n $ are equinumerous with 3-nonnesting set partitions of $ \{1，2，.，n\} $. In this paper, we confirm this conjecture by showing that 210-avoiding ascent sequences of length $ n $ are in bijection with 3-nonnesting set partitions of $ \…
We consider a variant of the game of Brussels Sprouts that, like Conway's original version, ends in a predetermined number of moves. We show that the endstates of the game are in natural bijection with noncrossing trees and that the game histories are in natural bijection with both parking functions and factorizations of a cycle of $S_n$.
For any integer $k\geq2$, we prove combinatorially the following Euler (binomial) transformation identity $$ \NC_{n+1}^{(k)}(t)=t\sum_{i=0}^n{n\choose i}\NW_{i}^{(k)}(t), $$ where $\NC_{m}^{(k)}(t)$ (resp.~$\NW_{m}^{(k)}(t)$) is the sum of weights, $t^\text{number of blocks}$, of partitions of $\{1,\ldots,m\}$ without $k$-crossings (resp.~enhanced $k$-crossings). The special $k=2$ and $t=1$ case, asserting the Euler transformation of Motzkin numbers are Catalan numbers, was discovered by Donaghey 1977. The result for $k=3$ and $t=1$, arising naturally in a recent study of pattern avoidance in …
Some combinatorics related to central binomial coefficients: Grand-Dyck paths, coloured noncrossing partitions and signed pattern avoiding permutations
We introduce two order relations on finite Coxeter groups which refine the absolute and the Bruhat order, and establish some of their main properties. In particular we study the restriction of these orders to noncrossing partitions and show that the intervals for these orders can be enumerated in terms of the cluster complex. The properties of our orders permit to revisit several results in Coxeter combinatorics, such as the Chapoton triangles and how they are related, the enumeration of reflections with full support, the bijections between clusters and noncrossing partitions.
Hypertrees and noncrossing trees are well-established objects in the combinatorics literature, but the hybrid notion of a noncrossing hypertree has received less attention. In this article I investigate the poset of noncrossing hypertrees as an induced subposet of the hypertree poset. Its dual is the face poset of a simplicial complex, one that can be identified with a generalized cluster complex of type $A$. The first main result is that this noncrossing hypertree complex is homeomorphic to a piecewise spherical complex associated with the noncrossing partition lattice and thus it has a natur…
在本文中，我们介绍了多项式时间算法，产生随机的3-不相交的划分和2-正规，3-不相交的划分具有均匀的概率。一个3-非交叉划分在其规范表示中不包含任何三个相互交叉的弧，并且如果后者不包含$(i,i+1)$形式的弧，则它是2-正则的。使用Chen的双射{ \it et al.} \cite{Chen,Reidys:08tan}，我们把3-不相交的划分和2-正则3-不相交的划分解释为受限制的广义摆动表.此外，我们解释的tableaux作为抽样路径的马尔可夫过程的形状，并得出其转移概率。
We introduce bijections between generalized type $A_n$ noncrossing partitions (that is, associated to arbitrary standard Coxeter elements) and fully commutative elements of the same type. The latter index the diagram basis of the classical Temperley-Lieb algebra, while for each choice of standard Coxeter element the corresponding noncrossing partitions also index a basis, given by the images in the Temperley-Lieb algebra of the simple elements of the dual Garside structure (associated to this choice of standard Coxeter element) of the Artin braid group on $n+1$ strands. We then show that our b…
There is a well-known bijection between parking functions of a fixed length and maximal chains of the noncrossing partition lattice which we can use to associate to each set of parking functions a poset whose Hasse diagram is the union of the corresponding maximal chains. We introduce a decomposition of parking functions based on the largest number omitted and prove several theorems about the corresponding posets. In particular, they share properties with the noncrossing partition lattice such as local self-duality, a nice characterization of intervals, a readily computable Möbius function, an…
Let $Q$ be an acyclic quiver and $w \geq 1$ be an integer. Let $\mathsf{C}_{-w} (\mathbf{k} Q)$ be the $(-w)$-cluster category of $\mathbf{k} Q$. We show that there is a bijection between simple-minded collections in $\mathsf{D}^b (\mathbf{k} Q)$ lying in a fundamental domain of $\mathsf{C}_{-w} (\mathbf{k} Q)$ and $w$-simple-minded systems in $\mathsf{C}_{-w} (\mathbf{k} Q)$. This generalises the same result of Iyama-Jin in the case that $Q$ is Dynkin. A key step in our proof is the observation that the heart $\mathsf{H}$ of a bounded t-structure in a Hom-finite, Krull-Schmidt, $\mathbf{k}$-l…
Noncrossing normal ordering for functions of boson operators
玻色子算符的函数的正常有序形式在许多情况下是重要的，特别是关于量子场论和量子光学。从Katriel的开创性工作开始[Lett. Nuovo Cimento，10（13）：565- 567，1974]，在过去的几年里，正常有序形式已经被证明具有丰富的组合结构，主要是凭借与划分理论的联系。在本文中，我们试图丰富这一联系。通过考虑非交叉分拆的线性表示，我们定义了非交叉正规序的概念。由于非交叉划分有许多意想不到的联系（例如，与自由概率的联系），非交叉正规序似乎是一个有趣的概念。我们明确地给出了函数（a^{r}（a^<$）^{s}）^{n}）和（a^{r}+（a^<$）^{s}）^{n}的非交叉正规序形式，以及各种特殊情况。我们能够建立这些功能，k元树和集的格路径之间的非交叉收缩的第一次双射。
In this paper we enumerate the number of $(k, r)$-Fuss-Schröder paths of type $λ$. Y. Park and S. Kim studied small Schröder paths with type $λ$. Generalizing the results to small $(k, r)$-Fuss-Schröder paths with type $λ$, we give a combinatorial interpretation for the number of small $(k, r)$-Fuss-Schröder paths of type $λ$ by using Chung-Feller style. We also give two sets of sparse noncrossing partitions of $[2(k + 1)n + 1]$ and $[2(k + 1)n + 2]$ which are in bijection with the set of all small and large, respectively, $(k, r)$-Fuss-Schröder paths of type $λ$.
