Schubert Varieties Research Articles

An effective decomposition theorem for Schubert varieties

Given a Schubert variety S contained in a Grassmannian Gk(Cl), we show how to obtain further information on the direct summands of the derived pushforward Rπ⁎QS˜ given by the application of the decomposition theorem to a suitable resolution of singularities π:S˜→S. As a by-product, Poincaré polynomial expressions are obtained along with an algorithm which computes the unknown terms in such expressions and which shows that the actual number of direct summands happens to be less than the number of supports of the decomposition.

The isomorphism problem for Grassmannian Schubert varieties

We prove that Schubert varieties in potentially different Grassmannians are isomorphic as varieties if and only if their corresponding Young diagrams are identical up to a transposition. We also discuss a generalization of this result to Grassmannian Richardson varieties. In particular, we prove that Richardson varieties in potentially different Grassmannians are isomorphic as varieties if their corresponding skew diagrams are semi-isomorphic as posets, and we conjecture the converse. Here, two posets are said to be semi-isomorphic if there is a bijection between their sets of connected components such that the corresponding components are either isomorphic or opposite.

LS algebras, valuations, and Schubert varieties

In this paper, we propose an algebraic approach via Lakshmibai–Seshadri (LS) algebras to establish a link between standard monomial theories, Newton–Okounkov bodies and valuations. This is applied to Schubert varieties, where this approach is compatible with the one using Seshadri stratifications in arXiv:2112.03776, showing that LS paths encode vanishing multiplicities with respect to the web of Schubert varieties.

On the Generating Function for Intervals in Young's Lattice

In this paper, we study a family of generating functions whose coefficients are polynomials that enumerate partitions in lower order ideals of Young's lattice. Our main result is that this family satisfies a rational recursion and are therefore rational functions. As an application, we calculate the asymptotic behavior of the cardinality of a lower order ideals for the "average" partition of fixed length and give a homological interpretation of this result in relation to Grassmannians and their Schubert varieties.

Poisson Trace Orders

Abstract The two main approaches to the study of irreducible representations of orders (via traces and Poisson orders) have so far been applied in a completely independent fashion. We define and study a natural compatibility relation between the two approaches leading to the notion of Poisson trace orders. It is proved that all regular and reduced traces are always compatible with any Poisson order structure. The modified discriminant ideals of all Poisson trace orders are proved to be Poisson ideals and the zero loci of discriminant ideals are shown to be unions of symplectic cores, under natural assumptions (maximal orders and Cayley–Hamilton algebras). A base change theorem for Poisson trace orders is proved. A broad range of Poisson trace orders are constructed based on the proved theorems: quantized universal enveloping algebras, quantum Schubert cell algebras and quantum function algebras at roots of unity, symplectic reflection algebras, 3D and 4D Sklyanin algebras, Drinfeld doubles of pre-Nichols algebras of diagonal type, and root of unity quantum cluster algebras.

Rigid toric matrix Schubert varieties

Fulton proves that the matrix Schubert variety overline{X_{pi }} cong Y_{pi } times mathbb {C}^q can be defined via certain rank conditions encoded in the Rothe diagram of pi in S_N. In the case where Y_{pi }:={{,textrm{TV},}}(sigma _{pi }) is toric (with respect to a (mathbb {C}^*)^{2N-1} action), we show that it can be described as a toric (edge) ideal of a bipartite graph G^{pi }. We characterize the lower dimensional faces of the associated so-called edge cone sigma _{pi } explicitly in terms of subgraphs of G^{pi } and present a combinatorial study for the first-order deformations of Y_{pi }. We prove that Y_{pi } is rigid if and only if the three-dimensional faces of sigma _{pi } are all simplicial. Moreover, we reformulate this result in terms of the Rothe diagram of pi .

A cluster structure on the coordinate ring of partial flag varieties

The main goal of this paper is to show that the (multi-homogeneous) coordinate ring of a partial flag variety C[G/PK−] contains a cluster algebra if G is any semisimple complex algebraic group. We use derivation properties and a special lifting map to prove that the cluster algebra structure A of the coordinate ring C[NK] of a Schubert cell constructed by Goodearl and Yakimov can be lifted, in an explicit way, to a cluster structure Aˆ living in the coordinate ring of the corresponding partial flag variety. Then we use a minimality condition to prove that the cluster algebra Aˆ is equal to C[G/PK−] after localizing some special minors.

A Harder–Narasimhan stratification of the -Grassmannian

We establish a Harder–Narasimhan formalism for modifications of $G$-bundles on the Fargues–Fontaine curve. The semi-stable stratum of the associated stratification of the ${B^+_{{\rm dR}}}$-Grassmannian coincides with the variant of the weakly admissible locus defined by Viehmann, and its classical points agree with those of the basic Newton stratum. When restricted to minuscule affine Schubert cells, the stratification corresponds to the Harder–Narasimhan stratification of Dat, Orlik and Rapoport. We also study basic geometric properties of the strata, and the relation to the Hodge–Newton decomposition.

Geometric properties of the Kazhdan–Lusztig Schubert basis

We study classes determined by the Kazhdan-Lusztig basis of the Hecke algebra in the $K$-theory and hyperbolic cohomology theory of flag varieties. We first show that, in $K$-theory, the two different choices of Kazhdan-Lusztig bases produce dual bases, one of which can be interpreted as characteristic classes of the intersection homology mixed Hodge modules. In equivariant hyperbolic cohomology, we show that if the Schubert variety is smooth, then the class it determines coincides with the class of the Kazhdan-Lusztig basis; this was known as the Smoothness Conjecture. For Grassmannians, we prove that the classes of the Kazhdan-Lusztig basis coincide with the classes determined by Zelevinsky's small resolutions. These properties of the so-called KL-Schubert basis show that it is the closest existing analogue to the Schubert basis for hyperbolic cohomology; the latter is a very useful testbed for more general elliptic cohomologies.

Algorithmic coincidence classification of mesh patterns

Permutations have connections to other mathematical objects such as Schubert varieties, sorting networks, and genome rearrangements. Often the connection is described in terms of patterns that are absent from the permutations. There can be ambiguity in this description, in the sense that the same subset of permutations can be defined with two different patterns. This is called coincidence and we focus on the coincidence of mesh patterns, one of the most descriptive version of patterns in permutations. We review and extend previous results on coincidence of mesh patterns. We introduce the notion of a force on a permutation pattern and apply it to the coincidence classification of mesh patterns, completing the classification up to size three. We also show that this concept can be used to enumerate classical permutation classes.

Conjectural Positivity of Chern–Schwartz–MacPherson Classes for Richardson Cells

Abstract Following some work of Aluffi–Mihalcea–Schürmann–Su for the CSM classes of Schubert cells and some elaborate computer calculations by Rimanyi and Mihalcea, I conjecture that the CSM classes of the Richardson cells expressed in the Schubert basis have nonnegative coefficients. This conjecture was principally motivated by a new product $\square $ coming from the Segre-SM classes in the cohomology of flag varieties (such that the associated Gr of this product is the standard cup product) and the conjecture that the structure constants of this new product $\square $ in the standard Schubert basis have alternating sign behavior. I prove that this conjecture on the sign of the structure constants of $\square $ would follow from my above positivity conjecture about the CSM classes of Richardson cells.

Relative Richardson varieties

AbstractA Richardson variety in a flag variety is an intersection of two Schubert varieties defined by transverse flags. We define and study relative Richardson varieties, which are defined over a base scheme with a vector bundle and two flags. To do so, we generalise transversality of flags to a relative notion, versality, that allows the flags to be non-transverse over some fibers. Relative Richardson varieties share many of the geometric properties of Richardson varieties. We generalise several geometric and cohomological facts about Richardson varieties to relative Richardson varieties. We also prove that the local geometry of a relative Richardson variety is governed, in a precise sense, by the two intersecting Schubert varieties, giving a generalisation, in the flag variety case, of a theorem of Knutson–Woo–Yong; we also generalise this result to intersections of arbitrarily many relative Schubert varieties. We give an application to Brill–Noether varieties on elliptic curves, and a conjectural generalisation to higher genus curves.

A Tableau Formula for Vexillary Schubert Polynomials in Type C

Ikeda-Mihalcea-Naruse's double Schubert polynomials [Adv. Math. 226 (2011)] represent the equivariant cohomology classes of Schubert varieties in the type C flag varieties. The goal of this paper is to obtain a new tableau formula of these polynomials associated to vexillary signed permutations introduced by Anderson-Fulton. To achieve that goal, we introduce flagged factorial (Schur) $Q$-functions, combinatorially defined functions in terms of marked shifted tableaux for flagged strict partitions, and prove their Schur-Pfaffian formula. As an application, we also obtain a new combinatorial formula of factorial $Q$-functions of Ivanov in which monomials bijectively corresponds to flagged marked shifted tableaux.

Harder-Narasimhan strata and 𝑝-adic period domains

We revisit the Harder-Narasimhan stratification on a minuscule p p -adic flag variety, by the theory of modifications of G G -bundles on the Fargues-Fontaine curve. We compare the Harder-Narasimhan strata with the Newton strata introduced by Caraiani-Scholze. As a consequence, we get further equivalent conditions in terms of p p -adic Hodge-Tate period domains for fully Hodge-Newton decomposable pairs. Moreover, we generalize these results to arbitrary cocharacters case by considering the associated B d R + B_{dR}^+ -affine Schubert varieties. Applying Hodge-Tate period maps, our constructions give applications to p p -adic geometry of Shimura varieties and their local analogues.

Permutation actions on Quiver Grassmannians for the equioriented cycle via GKM-theory

In our previous work, we equipped quiver Grassmannians for nilpotent representations of the equioriented cycle with an action of an algebraic torus. We show here that the equivariant cohomology ring is acted upon by a product of symmetric groups and we investigate this permutation action via GKM techniques. In the case of (type A) flag varieties, or Schubert varieties therein, we recover Tymoczko’s results on permutation representations.

On automorphisms of undirected Bruhat graphs

The undirected Bruhat graph \(\Gamma (u,v)\) has the elements of the Bruhat interval [u, v] as vertices, with edges given by multiplication by a reflection. Famously, \(\Gamma (e,v)\) is regular if and only if the Schubert variety \(X_v\) is smooth, and this condition on v is characterized by pattern avoidance. In this work, we classify when \(\Gamma (e,v)\) is vertex-transitive; surprisingly this class of permutations is also characterized by pattern avoidance and sits nicely between the classes of smooth permutations and self-dual permutations. This leads us to a general investigation of automorphisms of \(\Gamma (u,v)\) in the course of which we show that special matchings, which originally appeared in the theory of Kazhdan–Lusztig polynomials, can be characterized, for the symmetric and right-angled groups, as certain \(\Gamma (u,v)\)-automorphisms which are conjecturally sufficient to generate the orbit of e under \({{\,\textrm{Aut}\,}}(\Gamma (e,v))\).

Products of reflections in smooth Bruhat intervals

A permutation is called smooth if the corresponding Schubert variety is smooth. Gilboa and Lapid prove that in the symmetric group, multiplying the reflections below a smooth element $w$ in Bruhat order in a compatible order yields back the element $w$. We strengthen this result by showing that such a product in fact determines a saturated chain $e \to w$ in Bruhat order, and that this property characterizes smooth elements.

Mather classes of Schubert varieties via small resolutions

Mather classes of Schubert varieties via small resolutions

Grassmanniennes affines tordues sur les entiers

Abstract We generalize the works of Pappas–Rapoport–Zhu on twisted affine Grassmannians to the wildly ramified case under mild assumptions. This rests on a construction of certain smooth affine $\mathbb {Z}[t]$ -groups with connected fibers of parahoric type, motivated by previous work of Tits. The resulting $\mathbb {F}_p(t)$ -groups are pseudo-reductive and sometimes non-standard in the sense of Conrad–Gabber–Prasad, and their $\mathbb {F}_p [\hspace {-0,5mm}[ {t} ]\hspace {-0,5mm}] $ -models are parahoric in a generalized sense. We study their affine Grassmannians, proving normality of Schubert varieties and Zhu’s coherence theorem.

Stellahedral geometry of matroids

Abstract We use the geometry of the stellahedral toric variety to study matroids. We identify the valuative group of matroids with the cohomology ring of the stellahedral toric variety and show that valuative, homological and numerical equivalence relations for matroids coincide. We establish a new log-concavity result for the Tutte polynomial of a matroid, answering a question of Wagner and Shapiro–Smirnov–Vaintrob on Postnikov–Shapiro algebras, and calculate the Chern–Schwartz–MacPherson classes of matroid Schubert cells. The central construction is the ‘augmented tautological classes of matroids’, modeled after certain toric vector bundles on the stellahedral toric variety.