A polytope combinatorics for semisimple groups

Combinatorics and Genus of Tropical Intersections and Ehrhart Theory

Let $g_1, ..., g_k$ be tropical polynomials in $n$ variables with Newton polytopes $P_1, ..., P_k$. We study combinatorial questions on the intersection of the tropical hypersurfaces defined by $g_1, ..., g_k$, such as the $f$-vector, the number of unbounded faces and (in case of a curve) the genus. Our point of departure is Vigeland's work who considered the special case $k=n-1$ and where all Newton polytopes are standard simplices. We generalize these results to arbitrary $k$ and arbitrary Newton polytopes $P_1, ..., P_k$. This provides new formulas for the number of faces and the genus in t…

Torus actions and combinatorics of polytopes

An n-dimensional polytope P^n is called simple if exactly n codimension-one faces meet at each vertex. The lattice of faces of a simple polytope P^n with m codimension-one faces defines an arrangement of even-dimensional planes in R^{2m}. We construct a free action of the group R^{m-n} on the complement of this arrangement. The corresponding quotient is a smooth manifold Z_P invested with a canonical action of the compact torus T^m with the orbit space P^n. For each smooth projective toric variety M^{2n} defined by a simple polytope P^n with the given lattice of faces there exists a subgroup T…

论Arthur迹公式的组合学、凸多面体与环面簇

On Combinatorics of the Arthur Trace Formula, Convex Polytopes, and Toric Varieties

本文从扇与多面体出发构造组合截断函数，证明其积分的收敛性与多项式性，推广了Arthur迹公式、Ehrhart多项式及Langlands引理，并给出环面簇上的几何解释。

Ehrhart polynomials of polytopes and spectrum at infinity of Laurent polynomials

Gathering different results from singularity theory, geometry and combinatorics, we show that the spectrum at infinity of a tame Laurent polynomial counts lattice points in polytopes and we deduce an effective algorithm in order to compute the Ehrhart polynomial of a simplex containing the origin as an interior point.

超立方体与结合多面体之间的组合插值

A combinatorial interpolation between the hypercube and the associahedron

本文研究一族介于超立方体与结合多面体之间的多面体，给出其正规扇的弦图描述，推广佩尔数并渐近于卡塔兰数，且所有多面体均产生二元几何。

Birational Geometry of Toric Varieties

Toric geometry provides a bridge between algebraic geometry and combinatorics of fans and polytopes. For each polarized toric variety (X,L) we have associated a polytope P. In this thesis we use this correspondence to study birational geometry for toric varieties. To this end, we address subjects such as Minimal Model Program, Mori fiber spaces, and chamber structures on the cone of effective divisors. We translate some results from these theories to the combinatorics of polytopes and use them to get structure theorems on space of polytopes. In particular, we treat toric varieties known as 2-F…

扭曲仿射格拉斯曼流形中 Mirković-Vilonen 环的组合学

Combinatorics of Mirković-Vilonen cycles in twisted affine Grassmannians

本文研究扭曲仿射格拉斯曼流形中 MV 环的几何与组合结构，通过扭转映射给出 Lusztig 数据参数化，证明约化词变化由 Lusztig 变换支配，并得出矩多胞体与通常 MV 多胞体一致，适用于所有特殊抛物型子群。

相关均衡的组合数学

Combinatorics of Correlated Equilibria

本文从组合学角度研究相关均衡多面体，引入全维性区域并证明其为半代数集，利用有向拟阵分层刻画其组合类型，并证明一般(2×3)-博弈具有唯一的最大维数组合类型。

The Geometry and Combinatorics of Some Hessenberg Varieties Related to the Permutohedral Variety

We construct a concrete isomorphism from the permutohedral variety to the regular semisimple Hessenberg variety associated to the Hessenberg function $h_+(i)=i+1$, $1\le i\le n-1$. In the process of defining the isomorphism, we introduce a sequence of varieties which we call the prepermutohedral varieties. We first determine the toric structure of these varieties and compute the Euler characteristics and the Betti numbers using the theory of toric varieties. Then, we describe the cohomology of these varieties. We also find a natural way to encode the one-dimensional components of the cohomolog…

Favourable modules: Filtrations, polytopes, Newton-Okounkov bodies and flat degenerations

We introduce the notion of a favourable module for a complex unipotent algebraic group, whose properties are governed by the combinatorics of an associated polytope. We describe two filtrations of the module, one given by the total degree on the PBW basis of the corresponding Lie algebra, the other by fixing a homogeneous monomial order on the PBW basis. In the favourable case a basis of the module is parameterized by the lattice points of a normal polytope. The filtrations induce flat degenerations of the corresponding flag variety to its abelianized version and to a toric variety, the specia…

Enumeration of Gelfand-Cetlin type reduced words

The combinatorics of reduced words and commutation classes plays an important role in geometric representation theory. A string polytope is a lattice polytope associated to each reduced word of the longest element $w_0$ in the symmetric group which encodes the character of a certain irreducible representation of a Lie group of type $A$. In this paper, we provide a recursive formula for the number of reduced words of $w_0$ such that the corresponding string polytopes are combinatorially equivalent to a Gelfand-Cetlin polytope. The recursive formula involves the number of standard Young tableaux…