Positivity for cluster algebras

Parametrizations of $k$-Nonnegative Matrices: Cluster Algebras and $k$-Positivity Tests

A $k$-positive matrix is a matrix where all minors of order $k$ or less are positive. Computing all such minors to test for $k$-positivity is inefficient, as there are $\sum_{\ell=1}^k \binom{n}{\ell}^2$ of them in an $n\times n$ matrix. However, there are minimal $k$-positivity tests which only require testing $n^2$ minors. These minimal tests can be related by series of exchanges, and form a family of sub-cluster algebras of the cluster algebra of total positivity tests. We give a description of the sub-cluster algebras that give $k$-positivity tests, ways to move between them, and an altern…

Positivity and tameness in rank 2 cluster algebras

We study the relationship between the positivity property in a rank 2 cluster algebra, and the property of such an algebra to be tame. More precisely, we show that a rank 2 cluster algebra has a basis of indecomposable positive elements if and only if it is of finite or affine type. This statement disagrees with a conjecture by Fock and Goncharov.

球Hecke代数的预典范基的正性

Positivity of Pre-Canonical Bases for Spherical Hecke Algebras

该研究提出计算Kostka-Foulkes多项式的新算法，证明球Hecke代数预典范基间转换矩阵系数非负，确立正性猜想，还得到典范基原子分解算法，对应新的Schur正对称函数类。

来自轨道流形的量子簇代数的正性

Positivity for quantum cluster algebras from orbifolds

针对源自标记轨道流形的具任意系数与量子化的量子簇代数，提出了其量子簇变量关于任意量子种子的量子Laurent展开组合公式，并由此证明了该代数的正性。

Proof of the Kontsevich Non-Commutative Cluster Positivity Conjecture

We extend the Lee-Schiffler Dyck path model to give a proof of the Kontsevich non-commutative cluster positivity conjecture with unequal parameters.

Strong positivity for quantum theta bases of quantum cluster algebras

We construct "quantum theta bases," extending the set of quantum cluster monomials, for various versions of skew-symmetric quantum cluster algebras. These bases consist precisely of the indecomposable universally positive elements of the algebras they generate, and the structure constants for their multiplication are Laurent polynomials in the quantum parameter with non-negative integer coefficients, proving the quantum strong cluster positivity conjecture for these algebras. The classical limits recover the theta bases considered by Gross-Hacking-Keel-Kontsevich. Our approach combines the sca…

Positivity in Quantum Cluster Algebras and Flags of Valued Quiver Representations

In this paper we give a direct proof of the positivity conjecture for adapted quantum cluster variables. Moreover, our process allows one to explicitly compute formulas for all adapted cluster monomials and certain ordered products of adapted cluster monomials. In particular, we describe all cluster monomials in cluster algebras and quantum cluster algebras of rank 2. One may obtain similar formulas for all finite type cluster monomials. The above results are achieved by computing explicit set-theoretic decompositions of Grassmannians of subrepresentations in adapted valued quiver representati…

Affine Schubert classes, Schur positivity, and combinatorial Hopf algebras

We suggest the point of view that the Schubert classes of the affine Grassmannian of a simple algebraic group should be considered as Schur-positive symmetric functions. In particular, we give a geometric explanation of the Schur positivity of k-Schur functions (at t = 1). We also put this in the context of the theory of combinatorial Hopf algebras.

Positivity of Sylow branching coefficients of symmetric groups

In this article we investigate the positivity of Sylow branching coefficients for symmetric groups when $p = 3$. In particular, we complete the discussion begun by Giannelli and the author in arXiv:1712.02642 (J. Algebra) and developed in arXiv:1909.09446 (J. London Math. Soc.) concerning the case of odd primes.

图Laurent现象代数的正性

Positivity for graph Laurent phenomenon algebras

本文证明了Lam和Pylyavskyy提出的猜想：对于所有无环且无重边的有向图，任意种子簇下每个簇变量的Laurent展开式系数均为非负。

广义簇散射图的正性

Positivity of generalized cluster scattering diagrams

本文引入了极大Dyck路径上的紧标定这一组合对象，导出了秩2广义簇散射图墙函数的显式正性公式，并推广至高秩情形，证明了Laurent现象与Chekhov-Shapiro广义簇代数的强正性。

Positivity for quasi-cluster algebras

We generalise the expansion formulae of Musiker, Schiffler and Williams, obtained for cluster algebras from orientable surfaces, to a larger class of coefficients which we call principal laminations. In doing so, for any quasi-cluster algebra from a non-orientable surface, we are able to obtain expansion formulae for each cluster variable with respect to any initial quasi-triangulation $T$, and any choice of principal lamination. Moreover, generalising the `separation of additions' formula of Fomin and Zelevinsky, we settle a conjecture of Lam and Pylyavskyy in the setting of quasi-cluster alg…

Dual braid monoids, Mikado braids and positivity in Hecke algebras

We study the rational permutation braids, that is the elements of an Artin-Tits group of spherical type which can be written $x^{-1} y$ where $x$ and $y$ are prefixes of the Garside element of the braid monoid. We give a geometric characterization of these braids in type $A_n$ and $B_n$ and then show that in spherical types different from $D_n$ the simple elements of the dual braid monoid (for arbitrary choice of Coxeter element) embedded in the braid group are rational permutation braids (we conjecture this to hold also in type $D_n$).This property implies positivity properties of the polynom…

Interpolation polynomials, bar monomials, and their positivity

We prove a positivity result for interpolation polynomials that was conjectured by Knop and Sahi. These polynomials were first introduced by Sahi in the context of the Capelli eigenvalue problem for Jordan algebras, and were later shown to be related to Jack polynomials by Knop-Sahi and Okounkov-Olshanski. The positivity result proved here is an inhomogeneous generalization of Macdonald's positivity conjecture for Jack polynomials. We also formulate and prove the non-symmetric version of the Knop-Sahi conjecture, and in fact we deduce everything from an even stronger positivity result. This la…

A step towards the cluster positivity conjecture

We prove a conjecture of Kontsevich, which asserts that the iterations of the noncommutative rational map $F_r:(x,y)-->(xyx^{-1},(1+y^r)x^{-1})$ are given by noncommutative Laurent polynomials with nonnegative integer coefficients.

Proof of a positivity conjecture of M. Kontsevich on non-commutative cluster variables

We prove a conjecture of Kontsevich, which asserts that the iterations of the noncommutative rational map $F_r:(x,y)-->(xyx^{-1},(1+y^r)x^{-1})$ are given by noncommutative Laurent polynomials with nonnegative integer coefficients.

散射图、紧致分次与广义正性

Scattering diagrams, tight gradings, and generalized positivity

本文提出了一类新的组合对象“紧致分次”，借此给出了描述秩2一致散射图的正性公式，并证明了任意秩广义 cluster 散射图的墙函数系数为正，从而确立了广义 cluster algebras 的 Laurent 正性及其 theta 基的强正性。

Q-systems, Heaps, Paths and Cluster Positivity

We consider the cluster algebra associated to the $Q$-system for $A_r$ as a tool for relating $Q$-system solutions to all possible sets of initial data. We show that the conserved quantities of the $Q$-system are partition functions for hard particles on particular target graphs with weights, which are determined by the choice of initial data. This allows us to interpret the simplest solutions of the Q-system as generating functions for Viennot's heaps on these target graphs, and equivalently as generating functions of weighted paths on suitable dual target graphs. The generating functions tak…

秩三中的量子洛朗正性

Quantum Laurent positivity in rank three

本文证明了秩三斜对称化量子簇代数在任意相容量子化下的洛朗正性，方法基于突变无环正性。

Rooted Clusters for Graph LP Algebras

LP algebras, introduced by Lam and Pylyavskyy, are a generalization of cluster algebras. These algebras are known to have the Laurent phenomenon, but positivity remains conjectural. Graph LP algebras are finite LP algebras encoded by a graph. For the graph LP algebra defined by a tree, we define a family of clusters called rooted clusters. We prove positivity for these clusters by giving explicit formulas for each cluster variable. We also give a combinatorial interpretation for these expansions using a generalization of $T$-paths.

KP solitons, total positivity, and cluster algebras

Soliton solutions of the KP equation have been studied since 1970, when Kadomtsev and Petviashvili proposed a two-dimensional nonlinear dispersive wave equation now known as the KP equation. It is well-known that the Wronskian approach to the KP equation provides a method to construct soliton solutions. The regular soliton solutions that one obtains in this way come from points of the totally non-negative part of the Grassmannian. In this paper we explain how the theory of total positivity and cluster algebras provides a framework for understanding these soliton solutions to the KP equation. W…

Q-system Cluster Algebras, Paths and Total Positivity

In the first part of this paper, we provide a concise review of our method of solution of the $A_r$ Q-systems in terms of the partition function of paths on a weighted graph. In the second part, we show that it is possible to modify the graphs and transfer matrices so as to provide an explicit connection to the theory of planar networks introduced in the context of totally positive matrices by Fomin and Zelevinsky. As an illustration of the further generality of our method, we apply it to give a simple solution for the rank 2 affine cluster algebras studied by Caldero and Zelevinsky.

Positivity of the T-system cluster algebra

We give the path model solution for the cluster algebra variables of the $A_r$ $T$-system with generic boundary conditions. The solutions are partition functions of (strongly) non-intersecting paths on weighted graphs. The graphs are the same as those constructed for the $Q$-system in our earlier work, and depend on the seed or initial data in terms of which the solutions are given. The weights are "time-dependent" where "time" is the extra parameter which distinguishes the $T$-system from the $Q$-system, usually identified as the spectral parameter in the context of representation theory. The…

通过 $S_n$ 的群代数研究色度对称函数

Chromatic symmetric functions via the group algebra of $S_n$

本文借助对称群群代数刻画（超）森林色度对称函数的 Schur 系数，证明 Taylor 猜想，并引入带尖点色度对称函数以建立楔和的 Schur 正性判据。

用于图LP代数的蛇图（Snake Graphs）

Snake Graphs for Graph LP Algebras

本研究针对Lam和Pylyavskyy提出的图LP代数，利用蛇图的超图推广形式给出部分丛变量正性的组合证明，拓展了此前基于$T$-路径的相关研究成果。

An analogue of chromatic bases and $p$-positivity of skew Schur $Q$-functions

We investigate chromatic symmetric functions in the relation to the algebra $Γ$ of symmetric functions generated by Schur $Q$-functions. We construct natural bases of $Γ$ in terms of chromatic symmetric functions. We also consider the $p$-positivity of skew Schur $Q$-functions and find a class of $p$-positive ribbon Schur $Q$-functions, making a conjecture that they are \emph{all}. We include many concrete computational results that support our conjecture.

Discrete integrable systems, positivity, and continued fraction rearrangements

In this review article, we present a unified approach to solving discrete, integrable, possibly non-commutative, dynamical systems, including the $Q$- and $T$-systems based on $A_r$. The initial data of the systems are seen as cluster variables in a suitable cluster algebra, and may evolve by local mutations. We show that the solutions are always expressed as Laurent polynomials of the initial data with non-negative integer coefficients. This is done by reformulating the mutations of initial data as local rearrangements of continued fractions generating some particular solutions, that preserve…

长程差商、簇与Graham正性

Long range divided differences, clusters, and Graham-positivity

本文通过长程差商运算和组合算法，将环面轨道闭包的等变同调类展开为Graham正Schubert循环组合，推广了AJS-Billey公式并应用于曲线、轨道闭包及Richardson簇。

通过牛顿多面体研究簇代数中的递推公式、正性与多面体基

Recurrence formula, positivity and polytope basis in cluster algebras via Newton polytopes

本文通过牛顿多面体研究TSSS簇代数中F-多项式的性质，提出了簇变量Laurent表达式的递推构造，证明了多面体基的强正性，并构建了关键变量间的显式映射关系。

Cylindric symmetric functions and positivity

We introduce new families of cylindric symmetric functions as subcoalgebras in the ring of symmetric functions $Λ$ (viewed as a Hopf algebra) which have non-negative structure constants. Combinatorially these cylindric symmetric functions are defined as weighted sums over cylindric reverse plane partitions or - alternatively - in terms of sets of affine permutations. We relate their combinatorial definition to an algebraic construction in terms of the principal Heisenberg subalgebra of the affine Lie algebra $\mathfrak{\widehat{sl}}_n$ and a specialised cyclotomic Hecke algebra. Using Schur-We…

Stanley关于分配格Schur正性的猜想

Stanley's conjecture on the Schur positivity of distributive lattices

本文解决了Stanley于1998年提出的关于分配格的开放问题，通过构造两族反例分配格，分别证明了存在不好且非Schur正性的分配格，以及好但非Schur正性的分配格。

Beyond positivity in Ehrhart Theory

We study semigroup algebras arising from lattice polytopes, compute their volume polynomials (particularizing work of Hochster), and establish strong Lefschetz properties (generalizing work of the first three authors). This resolves several conjectures concerning unimodality properties of the $h^\ast$-polynomial of lattice polytopes arising within Ehrhart theory.

GKM spaces, and the signed positivity of the nabla operator

We show that the Frobenius character of the equivariant Borel-Moore homology of a certain positive $GL_n$-version of the unramified affine Springer fiber $Z_k$ studied by Goreski, Kottwitz and MacPherson is computed by the matrix coefficients of the $\nabla^k$-operator, which acts diagonally in the modified Macdonald basis. We do this by relating the combinatorial formula for the $\nabla^k$-operator we obtained in an earlier paper to the GKM paving of $Z_k$, and we give an algebraic presentation of the above homology as an explicit submodule of the Kostant-Kumar nil Hecke algebra. We then stud…

Sorting orders, subword complexes, Bruhat order and total positivity

In this note we construct a poset map from a Boolean algebra to the Bruhat order which unveils an interesting connection between subword complexes, sorting orders, and certain totally nonnegative spaces. This relationship gives a new proof of Björner and Wachs' result \cite{BW} that the proper part of Bruhat order is homotopy equivalent to the proper part of a Boolean algebra --- that is, to a sphere. We also obtain a geometric interpretation for sorting orders. We conclude with two new results: that the intersection of all sorting orders is the weak order, and the union of sorting orders is t…

Quantum cluster algebras from unpunctured triangulated surfaces: arbitrary coefficients and quantization

We study quantum cluster algebras from unpunctured surfaces with arbitrary coefficients and quantization. We first give a new proof of the Laurent expansion formulas for commutative cluster algebras from unpunctured surfaces, we then give the quantum Laurent expansion formulas for the quantum cluster algebras. Particularly, this gives a combinatorial proof of the positivity for such class of quantum cluster algebras.

电气网络、格拉斯曼流形与簇代数

Electrical networks, Grassmannians, and cluster algebras

研究行和为零的对称矩阵的循环全正性问题，通过在格拉斯曼流形坐标环上构造种子并与\(\mathcal{CM}_n\)比较，关联循环全正性与格拉斯曼流形正性，还证明\(\mathcal{LM}_n\)与电气网络非紧化空间坐标环同构。

Schur positivity of skew Schur function differences and applications to ribbons and Schubert classes

Some new relations on skew Schur function differences are established both combinatorially using Schützenberger's jeu de taquin, and algebraically using Jacobi-Trudi determinants. These relations lead to the conclusion that certain differences of skew Schur functions are Schur positive. Applying these results to a basis of symmetric functions involving ribbon Schur functions confirms the validity of a Schur positivity conjecture due to McNamara. A further application reveals that certain differences of products of Schubert classes are Schubert positive.

A positivity phenomenon in Elser's Gaussian-cluster percolation model

Veit Elser proposed a random graph model for percolation in which physical dimension appears as a parameter. Studying this model combinatorially leads naturally to the consideration of numerical graph invariants which we call \emph{Elser numbers} $\mathsf{els}_k(G)$, where $G$ is a connected graph and $k$ a nonnegative integer. Elser had proven that $\mathsf{els}_1(G)=0$ for all $G$. By interpreting the Elser numbers as Euler characteristics of appropriate simplicial complexes called \emph{nucleus complexes}, we prove that for all graphs $G$, they are nonpositive when $k=0$ and nonnegative for…

Non-Koszulness of operads and positivity of Poincaré series

We prove that the operad of mock partially associative $n$-ary algebras is not Koszul, as conjectured by the second and the third author in 2009, and utilise the Zeilberger's algorithm for hypergeometric summation to demonstrate that non-Koszulness of that operad cannot be established by hunting for negative coefficients in the inverse of its Poincaré series.

Proof of Lassalle's Positivity Conjecture on Schur Functions

In the study of Zeilberger's conjecture on an integer sequence related to the Catalan numbers, Lassalle proposed the following conjecture. Let $(t)_n$ denote the rising factorial, and let $Λ_{\mathbb{R}}$ denote the algebra of symmetric functions with real coefficients. If $φ$ is the homomorphism from $Λ_{\mathbb{R}}$ to $\mathbb{R}$ defined by $φ(h_n)={1}/{((t)_nn!)}$ for some $t>0$, then for any Schur function $s_λ$, the value $φ(s_λ)$ is positive. In this paper, we provide an affirmative answer to Lassalle's conjecture by using the Laguerre-Pólya-Schur theory of multiplier sequences.

广义簇代数的散射图

Scattering diagrams for generalized cluster algebras

本文为交换多项式分解为二项式的广义簇代数构造散射图，推广了经典簇散射图理论，并借此证明广义簇变量是 theta 函数且具有正性。

Regular cell complexes in total positivity

This paper proves a conjecture of Fomin and Shapiro that their combinatorial model for any Bruhat interval is a regular CW complex which is homeomorphic to a ball. The model consists of a stratified space which may be regarded as the link of an open cell intersected with a larger closed cell, all within the totally nonnegative part of the unipotent radical of an algebraic group. A parametrization due to Lusztig turns out to have all the requisite features to provide the attaching maps. A key ingredient is a new, readily verifiable criterion for which finite CW complexes are regular involving a…