Feifei Fan
A recent result of Papadakis-Petrotou shows that every simplicial sphere has the weak Lefschetz property in characteristic $2$. In this paper, we give a simpler proof of this result for PL-spheres by showing that the weak Lefschetz property in characteristic $2$ is preserved by bistellar moves. Several applications are given.
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We provide a simpler proof of the hard Lefschetz Theorem for face rings of PL spheres: While the algebraic theory remains the same, we replace the geometric constructions by Pachner's Theorem. This simplifies the reasoning for an important special case of the main result of the first author in arxiv:1812.10454, and already implies the McMullen g-conjecture for PL spheres, as well as the Grünbaum-Kalai-Sarkaria conjecture.
We prove the Weak Lefschetz theorem for simplicial PL-spheres. This result is weaker than the Hard Lefschetz theorem for more general spheres proved by Adiprasito (arXiv:1812.10454), but the proof here involves simple algebra and avoids the more complicated combinatorics.
The well known $g$-conjecture for homology spheres follows from the stronger conjecture that the face ring over the reals of a homology sphere, modulo a linear system of parameters, admits the strong-Lefschetz property. We prove that the strong-Lefschetz property is preserved under the following constructions on homology spheres: join, connected sum, and stellar subdivisions. The last construction is a step towards proving the $g$-conjecture for piecewise-linear spheres.
$f$-vectors of balanced simplicial complexes, flag spheres, and geometric Lefschetz decompositions
本文证明了均衡单纯复形的$f$-向量可展现类几何Lefschetz分解的布尔分解，基于Nevo--Petersen猜想将其与旗球面多项式的正定性及几何问题相联系，并指出分解次数未减半。
This short review is the result of a minicourse at the Sapienza University of Rome the author gave about the proof of the $g$-theorem. We review the hard Lefschetz theorem for simplicial spheres, as well as the theory at its core: perturbations of maps, biased Poincaré pairings and a cobordism argument that relates the Lefschetz property of a manifold to the Lefschetz property on its boundary. We also sketch an alternative argument based on edge-contractions.
In this paper we prove that a deformed tensor product of two Lefschetz algebras is a Lefschetz algebra. We then use this result in conjunction with some basic Schubert calculus to prove that the coinvariant ring of a finite reflection has the strong Lefschetz property.
Inspired by the works of Adiprasito, Babson, Nevo, and Murai on the $g$-conjecture, we consider different classes of PL-spheres and the relations between them. We focus on a certain class of spheres that is in the intersection of vertex-decomposable spheres (a concept due to Provan and Billera) and strongly edge-decomposable spheres (a concept due to Eran Nevo). The spheres in this class are exactly those vertex-decomposable ones for which Adiprasito's recent proof method works.
In this article, we consider the weighted generating function of matchings in the complete graph. We define an Artinian Gorenstein algebra as the quotient ring of a polynomial ring by the annihilator of the generating function. We show the strong Lefschetz property of the algebra.
Using a connection to lozenge tilings of triangular regions, we establish an easily checkable criterion that guarantees the weak Lefschetz property of a quotient by a monomial ideal. It is also shown that each such ideal also has a semistable syzygy bundle.
We prove the hard Lefschetz property for pseudomanifolds and cycles in any characteristic with respect to an appropriate Artinian reduction. The proof is a combination of Adiprasito's biased pairing theory and a generalization of a formula of Papadakis-Petrotou to arbitrary characteristic. In particular, we prove the Lefschetz theorem for doubly Cohen Macaulay complexes, solving a generalization of the g-conjecture due to Stanley. We also provide a simplified presentation of the characteristic 2 case, and generalize it to pseudomanifolds and cycles.
We study the weak Lefschetz property and the Hilbert function of level Artinian monomial almost complete intersections in three variables. Several such families are shown to have the weak Lefschetz property if the characteristic of the base field is zero or greater than the maximal degree of any minimal generator of the ideal. Two of the families have an interesting relation to tilings of hexagons by lozenges. This lends further evidence to a conjecture by Migliore, Miro-Roig, and the second author. Finally, using our results about the weak Lefschetz property, we show that the Hilbert function…
The weak Lefschetz property of artinian algebras associated to paths and cycles
本文研究由图边理想和变量平方定义的阿廷代数的弱莱夫谢茨性质，并对边数较少的图类（包括路径和圈）进行了分类。
MacMahon enumerated the plane partitions in an $a \times b \times c$ box. These are in bijection to lozenge tilings of a hexagon, to certain perfect matchings, and to families of non-intersecting lattice paths. In this work we consider more general regions, called triangular regions, and establish signed versions of the latter three bijections. Indeed, we use perfect matchings and families of non-intersecting lattice paths to define two signs of a lozenge tiling. A combinatorial argument involving a new method, called resolution of a puncture, then shows that the signs are in fact equivalent. …
We study the problem of whether an arbitrary codimension three graded artinian Gorenstein algebra has the Weak Lefschetz Property. We reduce this problem to checking whether it holds for all compressed Gorenstein algebras of odd socle degree. In the first open case, namely Hilbert function (1,3,6,6,3,1), we give a complete answer in every characteristic by translating the problem to one of studying geometric aspects of certain morphisms from $\mathbb P^2$ to $\mathbb P^3$, and Hesse configurations in $\mathbb P^2$.
Assume sigma is a face of a Gorenstein* simplicial complex D. We investigate the question of whether the Weak Lefschetz Property of the Stanley-Reisner ring k[D] (over an infinite field k) is equivalent to the same property of the Stanley-Reisner ring k[D_sigma] of the stellar subdivision D_sigma. We prove that this is the case if the dimension of sigma is big compared to the codimension.
In this work a combinatorial approach towards the weak Lefschetz property is developed that relates this property to enumerations of signed perfect matchings as well as to enumerations of signed families of non-intersecting lattice paths in certain triangular regions. This connection is used to study Artinian quotients by monomial ideals of a three-dimensional polynomial ring. Extending a main result in the recent memoir [Boij, Migliore, Miró-Roig, Nagel, Zanello], we completely classify the quotients of type two that have the weak Lefschetz property in characteristic zero. We also derive resu…
We introduce a natural correspondence between artinian monomial almost complete intersections in three variables and punctured hexagonal regions. We use this correspondence to investigate the algebras for the presence of the weak Lefschetz property. In particular, we relate the field characteristics in which such an algebra fails to have the weak Lefschetz property to the prime divisors of the enumeration of signed lozenge tilings of the associated punctured hexagonal region. On the one side this allows us to establish the weak Lefschetz property in many new cases. For numerous classes of punc…
Failure of the Lefschetz property for the Graphic Matroid
本文针对 Maeno 和 Numata 关于拟阵基生成多项式定义的 Artinian Gorenstein 代数具有强 Lefschetz 性质的猜想，构造了图拟阵反例，并通过构造高阶 Hessian 矩阵核中的非零元素证明其退化性。
Multigraded strong Lefschetz property for balanced simplicial complexes
本文引入多重分次强 Lefschetz 性质，证明特征2域上平衡同调球面的Stanley-Reisner环的Artin化满足该性质，从而得到旗h-数不等式，统一了单峰性与平衡广义下界不等式。
Roller Coaster Gorenstein algebras and Koszul algebras failing the weak Lefschetz property
受图论过山车定理启发，通过须状图定义的二次单项式代数的Nagata理想化构造，证明无约束希尔伯特级数的过山车Gorenstein代数存在，发现大量不满足弱Lefschetz性质的G-二次Gorenstein代数。
We show that an 'almost strong Lefschetz' property holds for the barycentric subdivision of a shellable complex. From this we conclude that for the barycentric subdivision of a Cohen-Macaulay complex, the $h$-vector is unimodal, peaks in its middle degree (one of them if the dimension of the complex is even), and that its $g$-vector is an $M$-sequence. In particular, the (combinatorial) $g$-conjecture is verified for barycentric subdivisions of homology spheres. In addition, using the above algebraic result, we derive new inequalities on a refinement of the Eulerian statistics on permutations,…
Repeated Lefschetz-like decompositions for flag doubly Cohen--Macaulay simplicial complexes and gamma vectors of flag spheres
本文提出了旗双重Cohen-Macaulay单纯复形h多项式的重复Lefschetz型分解，通过迭代双重悬垂和边细分净非负集合，建立了旗球面gamma向量与其几何构造的直接联系，并利用布尔与非布尔区别衡量旗球面偏离交叉多胞体边界的程度。
Independence polynomials and the weak Lefschetz property for tadpole graphs
本文证明蝌蚪图的独立多项式单峰并给出众数界，且分类了其边理想代数具有弱Lefschetz性质的参数对。
An artinian graded algebra, $A$, is said to have the Weak Lefschetz property (WLP) if multiplication by a general linear form has maximal rank in every degree. A vast quantity of work has been done studying and applying this property, touching on numerous and diverse areas of algebraic geometry, commutative algebra, and combinatorics. Amazingly, though, much of this work has a "common ancestor" in a theorem originally due to Stanley, although subsequently reproved by others. In this expository paper we describe the different directions in which research has moved starting with this theorem, an…
We study the positivity of complete intersections of nef classes. We first give a sufficient and necessary characterization on the complete intersection classes which have hard Lefschetz property on a compact complex torus, equivalently, in the linear case. In turn, this provides us new kinds of cohomology classes which have Hodge-Riemann property or hard Lefschetz property on an arbitrary compact Kähler manifold. We also give a complete characterization on when the complete intersection classes are non-vanishing on an arbitrary compact Kähler manifold. Both characterizations are given by the …
Subarrangements of type A: the weak Lefschetz property of the Artinian Orlik-Terao algebra
本文研究了图排列的Artinian Orlik-Terao代数的弱Lefschetz性质（WLP），发现即使对于弦图（产生Koszul代数）的情况，WLP有时也会失效；而通过分析状态多面体可知，当所有可能的初始理想都失效时，WLP仍可能成立。更一般地，对于任何具有张量积分解的代数，我们构造了乘法映射的核中的标准元素，改进了文献中的先前结果。
The weak Lefschetz property of whiskered graphs
本文研究带须图对应的Artinian水平代数的弱莱夫谢茨性质，借助Dao-Nair与Hausel的结果证明了特征非2及特征零时乘法映射的秩性质，且说明特征零情形的结论是最优的。
Strong Lefschetz property of the Kirchhoff algebra of the complete tripartite graph $K_{1,1,n}$
本文证明完全三部图$K_{1,1,n}$的圈拟阵基生成多项式对应的基尔霍夫代数，在特征0域上具有强莱夫谢茨性质，证明结合了戈特利布-威尔逊定理与对称群表示论。
On the Lefschetz Property for quotients by monomial ideals containing squares of variables
该研究通过单纯复形Δ刻画Artinian单项式代数A(Δ)的弱Lefschetz性质，补充了已有工作，给出2维伪流形的完整刻画，并构造出不满足该性质的Artinian Gorenstein代数。
We characterize the monomial complete intersections in three variables satisfying the Weak Lefschetz Property (WLP), as a function of the characteristic of the base field. Our result presents a surprising, and still combinatorially obscure, connection with the enumeration of plane partitions. It turns out that the rational primes p dividing the number, M(a,b,c), of plane partitions contained inside an arbitrary box of given sides a,b,c are precisely those for which a suitable monomial complete intersection (explicitly constructed as a bijective function of a,b,c) fails to have the WLP in chara…
We introduce Artinian Gorenstein algebras defined by the face posets of regular polyhedra. We consider the strong Lefschetz property and Hodge--Riemann relation for the algebras. We show the strong Lefschetz property of the algebras for all Platonic solids. On the other hand, for some Platonic solids, we show that the algebras do not satisfy the Hodge--Riemann relation with respect to some strong Lefschetz elements.
Anari, Gharan, and Vinzant proved (complete) log-concavity of the basis generating functions for all matroids. From the viewpoint of combinatorial Hodge theory, it is natural to ask whether these functions are "strictly" log-concave for simple matroids. In this paper, we show this strictness for simple graphic matroids, that is, we show that Kirchhoff polynomials of simple graphs are strictly log-concave. Our key observation is that the Kirchhoff polynomial {of a complete graph} can be seen as the (irreducible) relative invariant of a certain prehomogeneous vector space, which may be independe…
In 1971, McMullen and Walkup posed the following conjecture, which is called the generalized lower bound conjecture: If $P$ is a simplicial $d$-polytope then its $h$-vector $(h_0,h_1,...,h_d)$ satisfies $h_0 \leq h_1 \leq ... \leq h_{\lfloor \frac d 2 \rfloor}$. Moreover, if $h_{r-1}=h_r$ for some $r \leq \frac d 2$ then $P$ can be triangulated without introducing simplices of dimension $\leq d-r$. The first part of the conjecture was solved by Stanley in 1980 using the hard Lefschetz theorem for projective toric varieties. In this paper, we give a proof of the remaining part of the conjecture…
Coinvariant stresses, Lefschetz properties and random complexes
本文利用 Herzog 和 Moradi 的参数系结果，将几何复形的协变应力与单项式理想的 Lefschetz 性质联系起来，推广了 WLP 失效的经典结论，并在随机复形模型中给出概率推论。
Gröbner bases, resolutions, and the Lefschetz properties for powers of a general linear form in the squarefree algebra
研究几乎完全交理想$(x_1^2,\dots,x_n^2,(x_1+\cdots+x_n)^k)$的格罗伯纳基、勒夫歇茨性质及Betti数，利用组合结构证明弱勒夫歇茨性质，并构造极小自由解析。
The weak Lefschetz properties of artinian monomial algebras associated to certain tadpole graphs
该研究围绕与简单图相关的阿廷单项式代数$A(G)$的弱莱夫谢茨性质展开，对部分满足或不满足该性质的蝌蚪图进行了分类，推进了相关代数性质的图类刻画研究。
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