Base subsets of polar Grassmannians

Let Δ be a thick building of type Xn=Cn,Dn. Let also Gk be the Grassmannian of k-dimensional singular subspaces of the associated polar space Π (of rank n). We write Gk for the corresponding shadow space of type Xn,k. Every bijective transformation of Gk which maps base subsets to base subsets (the shadows of apartments) is a collineation of Gk, and it is induced by a collineation of Π if n≠4 or k≠1.     

Base subsets in symplectic Grassmannians of small indices

Let V and V′ be n-dimensional vector spaces. Let also Ω and Ω′ be non-degenerate symplectic forms defined on V and V′, respectively. Denote by Π and Π′ the projective spaces associated with V and V′. We put for the sets of k-dimensional totally isotropic subspaces of Π and Π′. We study mappings of BF03322901 which transfer base subsets to base subsets and show that such mappings are induced by strong embeddings of Π to Π′ if 3k + 3 ≤ n.     

Differential geometry of curves in Lagrange Grassmannians with given Young diagram

Curves in Lagrange Grassmannians appear naturally in the intrinsic study of geometric structures on manifolds. By a smooth geometric structure on a manifold we mean any submanifold of its tangent bundle, transversal to the fibers. One can consider the time-optimal problem naturally associated with a geometric structure. The Pontryagin extremals of this optimal problem are integral curves of certain Hamiltonian system in the cotangent bundle. The dynamics of the fibers of the cotangent bundle w.r.t. this system along an extremal is described by certain curve in a Lagrange Grassmannian, called Jacobi curve of the extremal. Any symplectic invariant of the Jacobi curves produces the invariant of the original geometric structure. The basic characteristic of a curve in a Lagrange Grassmannian is its Young diagram. The number of boxes in its kth column is equal to the rank of the kth derivative of the curve (which is an appropriately defined linear mapping) at a generic point. We will describe the construction of the complete system of symplectic invariants for parameterized curves in a Lagrange Grassmannian with given Young diagram. It allows to develop in a unified way local differential geometry of very wide classes of geometric structures on manifolds, including both classical geometric structures such as Riemannian and Finslerian structures and less classical ones such as sub-Riemannian and sub-Finslerian structures, defined on nonholonomic distributions.     

Caps Embedded in Grassmannians

This paper is concerned with constructing caps embedded in line Grassmannians. In particular, we construct a cap of size q³ +2q²+1 embedded in the Klein quadric of PG(5,q) for even q, and show that any cap maximally embedded in the Klein quadric which is larger than this one must have size equal to the theoretical upper bound, namely q³+2q²+q+2. It is not known if caps achieving this upper bound exist for even q > 2.     

Desingularization of quiver Grassmannians for Dynkin quivers

A desingularization of arbitrary quiver Grassmannians for representations of Dynkin quivers is constructed in terms of quiver Grassmannians for an algebra derived equivalent to the Auslander algebra of the quiver.     

Lines in Quantum Grassmannians

We study quantum lines in quantum grassmannians and prove that there are only finitely many corresponding to lines in usual grassmannians fixed by a maximal torus.     

Root games on Grassmannians

We recall the root game, introduced in [8], which gives a fairly powerful sufficient condition for non-vanishing of Schubert calculus on a generalised flag manifold G/B. We show that it gives a necessary and sufficient rule for non-vanishing of Schubert calculus on Grassmannians. In particular, a Littlewood-Richardson number is non-zero if and only if it is possible to win the corresponding root game. More generally, the rule can be used to determine whether or not a product of several Schubert classes on Gr _l (ℂ ⁿ ) is non-zero in a manifestly symmetric way. Finally, we give a geometric interpretation of root games for Grassmannian Schubert problems. Research partially supported by an NSERC scholarship.     

Grassmannians of symplectic subspaces

We study the geometry of the Grassmannians of symplectic subspaces in a symplectic vector space. We construct symplectic twistor spaces by the symplectic quotient construction and use them to describe the symplectic geometry of the symplectic Grassmannians.     

Range characterization of the cosine transform on higher Grassmannians

We characterize the range of the cosine transform on real Grassmannians in terms of the decomposition under the action of the special orthogonal group SO(n). We also give a geometric interpretation of this image in terms of valuations. In addition, we discuss the non-Archimedean analogues.     

Desingularizations of quiver Grassmannians via graded quiver varieties

Inspired by recent work of Cerulli, Feigin and Reineke on desingularizations of quiver Grassmannians of representations of Dynkin quivers, we obtain desingularizations in considerably more general situations and in particular for Grassmannians of modules over iterated tilted algebras of Dynkin type. Our desingularization map is constructed from Nakajima's desingularization map for graded quiver varieties.     

Symplectic subspaces of symplectic Grassmannians

Let $V$ be a non-degenerate symplectic space of dimension $2n$ over the field $\mathbb{F}$ and for a natural number $l<n$ denote by $C_l\left(V\right)$ the incidence geometry whose points are the totally isotropic $l$-dimensional subspaces of $V$. Two points $U,W$ of $C_l\left(V\right)$ will be collinear when $W\subset U^{\perp }$ and $dim(U\cap W)=l-1$ and then the line on $U$ and $W$ will consist of all the $l$-dimensional subspaces of $U+W$ which contain $U\cap W$. The isomorphism type of this geometry is denoted by $C_{n,l}\left(\mathbb{F}\right)$. When $\mathrm{char}\left(\mathbb{F}\right)\ne 2$ we classify subspaces $S$ of $C_l\left(\mathbb{F}\right)$ where $S\cong C_{m,k}\left(\mathbb{F}\right)$.     

Invariant points of maps between Grassmannians

We show that for a very large class of integers and any map between Grassmannians, there is some -plane of which is mapped into a subspace of itself.

     

Cup products in Grassmannians

This note calculates the height of the first Stiefel-Whitney class in the cohomology of the real Grassmannians and determines the length of the longest nontrivial cup-product in $\text{H}{}^1\text{(G}{}_{\mathrm{k}}\text{(R}{}^{\mathrm{n+k}}\text{);Z}{}_2\text{) (k?n)}$ with k?4.     

Gromov-Witten invariants on Grassmannians

We prove that any three-point genus zero Gromov-Witten invariant on a type Grassmannian is equal to a classical intersection number on a two-step flag variety. We also give symplectic and orthogonal analogues of this result; in these cases the two-step flag variety is replaced by a sub-maximal isotropic Grassmannian. Our theorems are applied, in type , to formulate a conjectural quantum Littlewood-Richardson rule, and in the other classical Lie types, to obtain new proofs of the main structure theorems for the quantum cohomology of Lagrangian and orthogonal Grassmannians.

     

Semi-parallel real hypersurfaces in complex two-plane Grassmannians

We prove that there does not exist any semi-parallel real hypersurface in complex two-plane Grassmannians. With this result, the nonexistence of recurrent real hypersurfaces in complex two-plane Grassmannians can also be proved.     

Smoothness of isotopy for symplectic pairs

Let M be a 2n-dimensional smooth manifold with a symplectic pair which is a pair of closed 2-forms of constant ranks with complementary kernel foliations. Similar to Moser's stability theorem for symplectic forms, one desires to establish a stability theorem for symplectic pairs. Some sufficient and necessary conditions are obtained by Bande, Ghiggini and Kotschick. In this article, we consider a technical problem relating to the stability theorem. To complete the proof of the stability theorem for symplectic pairs, we verify the smoothness of the isotopy which is ignored in the literature. The Hodge theory for Riemannian foliation is crucial to our discussion.